Revert "Add String.titlecase/2 (#10008)"

Unicode says "HH" is not titlecased, which means we need
to downcase the rest of the string, which is the behaviour
we already have in capitalize.

This reverts commit 3be3afb.

Author: josevalim

lib/elixir/lib/string.ex

@@ -838,23 +838,16 @@ defmodule String do
   Converts the first character in the given string to
   uppercase and the remainder to lowercase according to `mode`.
 
-  `mode` may be `:default`, `:ascii` or `:greek`. The `:default`
-  mode considers all non-conditional transformations outlined in
-  the Unicode standard. `:ascii` lowercases only the letters A to Z.
-  `:greek` includes the context sensitive mappings found in Greek.
-
-  More precisely, it converts the first character to titlecase.
-  If you want to convert only the first character without downcasing
-  the rest of the string, see `titlecase/2`.
+  `mode` may be `:default`, `:ascii` or `:greek`. The `:default` mode considers
+  all non-conditional transformations outlined in the Unicode standard. `:ascii`
+  lowercases only the letters A to Z. `:greek` includes the context sensitive
+  mappings found in Greek.
 
   ## Examples
 
       iex> String.capitalize("abcd")
       "Abcd"
 
-      iex> String.capitalize("OS")
-      "Os"
-
       iex> String.capitalize("fin")
       "Fin"
 
@@ -875,54 +868,6 @@ defmodule String do
     char <> downcase(rest, mode)
   end
 
-  @doc """
-  Converts only the first character in the given string to
-  uppercase according to `mode`.
-
-  `mode` may be `:default`, `:ascii` or `:greek`. The `:default`
-  mode considers all non-conditional transformations outlined in
-  the Unicode standard. `:ascii` lowercases only the letters A to Z.
-  `:greek` includes the context sensitive mappings found in Greek.
-
-  More precisely, it converts the first character to titlecase,
-  according to the Unicode Standard. This function makes no
-  attempt to titlecase each word, as those are often language
-  and locale specific.
-
-  If you want to uppercase the first character and downcase the
-  rest of the string, see `capitalize/2`.
-
-  ## Examples
-
-      iex> String.titlecase("abcd")
-      "Abcd"
-
-      iex> String.titlecase("OS")
-      "OS"
-
-      iex> String.titlecase("fin")
-      "Fin"
-
-      iex> String.titlecase("olá")
-      "Olá"
-
-  """
-  @spec titlecase(t, :default | :ascii | :greek) :: t
-  def titlecase(string, mode \\ :default)
-
-  def titlecase(<<char, rest::binary>>, :ascii) when char >= ?a and char <= ?z do
-    <<char - 32, rest::binary>>
-  end
-
-  def titlecase(string, :ascii) when is_binary(string) do
-    string
-  end
-
-  def titlecase(string, mode) when is_binary(string) do
-    {char, rest} = String.Casing.titlecase_once(string, mode)
-    char <> rest
-  end
-
   @doc false
   @deprecated "Use String.trim_trailing/1 instead"
   defdelegate rstrip(binary), to: String.Break, as: :trim_trailing

lib/elixir/test/elixir/string_test.exs

@@ -210,41 +210,6 @@ defmodule StringTest do
     assert String.capitalize("aáA", :ascii) == "Aáa"
   end
 
-  test "titlecase/1" do
-    assert String.titlecase("") == ""
-    assert String.titlecase("abc") == "Abc"
-    assert String.titlecase("ABC") == "ABC"
-    assert String.titlecase("c b a") == "C b a"
-    assert String.titlecase("1ABc") == "1ABc"
-    assert String.titlecase("_aBc1") == "_aBc1"
-    assert String.titlecase(" aBc1") == " aBc1"
-  end
-
-  test "titlecase/1 with UTF-8" do
-    assert String.titlecase("àáâ") == "Àáâ"
-    assert String.titlecase("ÀÁÂ") == "ÀÁÂ"
-    assert String.titlecase("âáàÕ") == "ÂáàÕ"
-    assert String.titlecase("ÂÁÀõ") == "ÂÁÀõ"
-    assert String.titlecase("òóôõö") == "Òóôõö"
-    assert String.titlecase("ÒÓÔÕÖ") == "ÒÓÔÕÖ"
-    assert String.titlecase("fin") == "Fin"
-  end
-
-  test "titlecase/2 with ascii" do
-    assert String.titlecase("àáâ", :ascii) == "àáâ"
-    assert String.titlecase("aáA", :ascii) == "AáA"
-  end
-
-  test "titlecase/1 with greek final sigma" do
-    assert String.titlecase("σ") == "Σ"
-    assert String.titlecase("μΜσΣ") == "ΜΜσΣ"
-  end
-
-  test "titlecase/2 with greek final sigma" do
-    assert String.titlecase("σ", :greek) == "Σ"
-    assert String.titlecase("σ ςΠ", :greek) == "Σ ςΠ"
-  end
-
   test "replace_leading/3" do
     assert String.replace_leading("aa abc   ", "a", "b") == "bb abc   "
     assert String.replace_leading("__ abc   ", "_", "b") == "bb abc   "