Drop Scala2compat language feature

It's been replaced a while ago by `-source 3.0-migration`

Author: odersky

compiler/src/dotty/tools/dotc/Driver.scala

@@ -75,8 +75,6 @@ class Driver {
     inContext(ictx) {
       if !ctx.settings.YdropComments.value || ctx.mode.is(Mode.ReadComments) then
         ictx.setProperty(ContextDoc, new ContextDocstrings)
-      if Feature.enabledBySetting(nme.Scala2Compat) then
-        report.warning("-language:Scala2Compat will go away; use -source 3.0-migration instead")
       val fileNames = CompilerCommand.checkUsage(summary, sourcesRequired)
       fromTastySetup(fileNames, ctx)
     }

compiler/src/dotty/tools/dotc/config/Feature.scala

@@ -82,8 +82,7 @@ object Feature:
       case Some(v) => v
       case none => sourceVersionSetting
 
-  def migrateTo3(using Context): Boolean =
-    sourceVersion == `3.0-migration` || enabledBySetting(nme.Scala2Compat)
+  def migrateTo3(using Context): Boolean = sourceVersion == `3.0-migration`
 
   /** If current source migrates to `version`, issue given warning message
    *  and return `true`, otherwise return `false`.

compiler/src/dotty/tools/dotc/core/StdNames.scala

@@ -386,7 +386,6 @@ object StdNames {
     val Ref: N                  = "Ref"
     val RootPackage: N          = "RootPackage"
     val RootClass: N            = "RootClass"
-    val Scala2Compat: N         = "Scala2Compat"
     val Select: N               = "Select"
     val Shape: N                = "Shape"
     val StringContext: N        = "StringContext"

library/src/scalaShadowing/language.scala

@@ -225,9 +225,6 @@ object language {
     object genericNumberLiterals
   }
 
-  /** Where imported, a backwards compatibility mode for Scala2 is enabled */
-  object Scala2Compat
-
   /** Where imported, auto-tupling is disabled */
   object noAutoTupling