go/types, types2: consistently report "duplicate method" error in go1.13
Go 1.13 is not supported anymore, but this CL removes an unnecessary
check and in turn fixes an old bug.
Fixes #66285.
Change-Id: I15ee1712b31f8ac8c915f18410d99cbf44334d35
Reviewed-on: https://go-review.googlesource.com/c/go/+/571058 Reviewed-by: Robert Griesemer <gri@google.com> Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>