Remove compatibility with old TASTY

Not needed since we bumped the TASTY major version.

Author: smarter

compiler/src/dotty/tools/dotc/core/tasty/TastyUnpickler.scala

@@ -64,7 +64,6 @@ class TastyUnpickler(reader: TastyReader) {
         val result = readName().toTypeName
         val params = until(end)(readName().toTypeName)
         var sig = Signature(params, result)
-        if (sig == Signature.NotAMethod) sig = Signature.NotAMethod // needed temporarily, as long as we read old tasty
         original.withSig(sig).asInstanceOf[TermName]
       case _ =>
         simpleNameKindOfTag(tag)(readName())