cmd/compile/internal/types2: remove Config.AcceptMethodTypeParams flag

Type parameters for methods are not part of the accepted language,
but maintaining the code for type-checking them ensures regularity
of the type checker implementation. For now, keep the flag internally,
disabled by default. The flag is set when running tests.

Change-Id: Ic99934bd00bd2608dc1178e4131f46dd1507f0f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/307214


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>