Revert "span: add span_extend_to_line helper"

This reverts commit 97edb9f.

Author: davidtwco

compiler/rustc_span/src/source_map.rs

@@ -734,11 +734,6 @@ impl SourceMap {
         sp
     }
 
-    /// Extends the given `Span` to contain the entire line it is on.
-    pub fn span_extend_to_line(&self, sp: Span) -> Span {
-        self.span_extend_to_prev_char(self.span_extend_to_next_char(sp, '\n', true), '\n', true)
-    }
-
     /// Given a `Span`, tries to get a shorter span ending before the first occurrence of `char`
     /// `c`.
     pub fn span_until_char(&self, sp: Span, c: char) -> Span {