Revert "go/types: keep generic methods out of method sets"

This reverts commit 2e47ef62c27e90d96e58ddb6c204ceb68861e509.

Reason for revert: Broken builders.

Change-Id: I73bdb78a6258d64c5eb194fb8b3caa77f4e3f923
Reviewed-on: https://go-review.googlesource.com/c/go/+/785740
Reviewed-by: Keith Randall <khr@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>