Revert "Special error message for extension (...) with"

This reverts commit 1173fcc.

Author: odersky

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3619,11 +3619,8 @@ object Parsers {
         isUsingClause(extParams)
       do ()
       leadParamss ++= paramClauses(givenOnly = true, numLeadParams = nparams)
-      if in.token == WITH then
-        syntaxError(
-          i"""No `with` expected here.
-             |
-             |An extension clause is simply followed by one or more method definitions.""")
+      if in.token == COLON then
+        syntaxError("no `:` expected here")
         in.nextToken()
       val methods =
         if isDefIntro(modifierTokens) then

tests/neg/extension-colon.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/extension-colon.scala:1:18 -------------------------------------------------------------------------
+1 |extension (x: Int): // error
+  |                  ^
+  |                  no `:` expected here

tests/neg/extension-colon.scala

@@ -0,0 +1,2 @@
+extension (x: Int): // error
+  def foo = x