Simplify erased parameter syntax

erased is not always a modifier on the parameter name. The syntax `erased Type` is dropped.
There's not really a need for this syntactic inconsistency.

Author: odersky

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

@@ -1631,7 +1631,7 @@ object Parsers {
                     () => funParam(in.offset, imods))
                 case t =>
                   def funArg() =
-                    addErased()
+                    erasedArgs.addOne(false)
                     funArgType()
                   commaSeparatedRest(t, funArg)
           accept(RPAREN)