cmd/compile: report type parameter error for methods only once

Move switch to enable method type parameters entirely
to the parser, by adding the mode AllowMethodTypeParams.
Ensure that the error messages are consistent.
Remove unnecessary code in the type checker.

Fixes #50317.

Change-Id: I4f3958722400bdb919efa4c494b85cf62f4002bb
Reviewed-on: https://go-review.googlesource.com/c/go/+/376054


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>