cmd/compile/internal/syntax: remove generic-specific parse modes

cmd/compile/internal/syntax: remove generic-specific parse modes

Generics have landed; we cannot revert the syntax anymore. Remove
ability to choose between non-generic and generic code. Also remove
mode to enable method type parameters. Adjust code accordingly.

Also remove a couple of TODOs that are not relevant anymore.

Remove tests from types2 which were focussed on method type parameters,
make types2 and go/types tests match up where there was a difference in
this regard.

Change-Id: I989bdcb19eea7414214af739187fa013a044295d
Reviewed-on: https://go-review.googlesource.com/c/go/+/396634
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>