Revert "Disallow direct unqualified mentions of extension methods."

Partial revert of the tests that had to be changed to satisfy the restriction
which is now dropped again.

Author: odersky

tests/pos/i7401.scala

@@ -7,5 +7,5 @@ object Test {
   x.foo(4)
   x.foo
   Test.foo(x)(4)
-  this.foo(x)
+  foo(x)
 }

tests/pos/reference/extension-methods.scala

@@ -118,6 +118,6 @@ object ExtMethods:
       if exponent == 0 then 1 else x * (x ** (exponent - 1))
 
   import DoubleOps.{**}
-  assert(2.0 ** 3 == DoubleOps.**(2.0)(3))
+  assert(2.0 ** 3 == **(2.0)(3))
 
 end ExtMethods

tests/run-macros/i6201/macro_1.scala

@@ -1,15 +1,13 @@
 import scala.quoted._
 
-object M:
+extension (inline x: String) inline def strip: String =
+  ${ stripImpl('x) }
 
-  extension (inline x: String) inline def strip: String =
-    ${ stripImpl('x) }
+def stripImpl(x: Expr[String])(using qctx: QuoteContext) : Expr[String] =
+  Expr(x.unliftOrError.stripMargin)
 
-  def stripImpl(x: Expr[String])(using qctx: QuoteContext) : Expr[String] =
-    Expr(x.unliftOrError.stripMargin)
+inline def isHello(inline x: String): Boolean =
+  ${ isHelloImpl('x) }
 
-  inline def isHello(inline x: String): Boolean =
-    ${ isHelloImpl('x) }
-
-  def isHelloImpl(x: Expr[String])(using qctx: QuoteContext) : Expr[Boolean] =
-    if (x.unliftOrError == "hello") Expr(true) else Expr(false)
+def isHelloImpl(x: Expr[String])(using qctx: QuoteContext) : Expr[Boolean] =
+  if (x.unliftOrError == "hello") Expr(true) else Expr(false)

tests/run-macros/i6201/test_2.scala

@@ -1,7 +1,6 @@
 object Test {
-  import M._
   def main(args: Array[String]): Unit = {
-    assert(isHello(M.strip("hello")))
-    assert(!isHello(M.strip("bonjour")))
+    assert(isHello(strip("hello")))
+    assert(!isHello(strip("bonjour")))
   }
 }

tests/run/extension-methods.scala

@@ -2,7 +2,7 @@ object Test extends App {
 
   extension (x: Int) def em: Boolean = x > 0
 
-  assert(1.em == this.em(1))
+  assert(1.em == em(1))
 
   case class Circle(x: Double, y: Double, radius: Double)