[dev.typeparams] cmd/compile/internal/types2: remove MethodSet code - not used by types2

We can always re-introduce it if we decide to make use of it.

Change-Id: Ia939fdae978568edc58e21d1af732c6137744aab
Reviewed-on: https://go-review.googlesource.com/c/go/+/285678


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>