Revert: Implementations of mixin getters are never ParamAccessors

(reverted from commit 2c55900)

Mixin should drop the ParamAccessor flag anyway in traits, so there is no
need to mask it when creating implementations of trait definitions.

Author: odersky

src/dotty/tools/dotc/transform/MixinOps.scala

@@ -17,7 +17,7 @@ class MixinOps(cls: ClassSymbol, thisTransform: DenotTransformer)(implicit ctx:
     val res = member.copy(
       owner = cls,
       name = member.name.stripScala2LocalSuffix,
-      flags = member.flags &~ (Deferred | ParamAccessor),
+      flags = member.flags &~ Deferred,
       info = cls.thisType.memberInfo(member)).enteredAfter(thisTransform).asTerm
     res.addAnnotations(member.annotations)
     res