go/types, types2: simplify missingMethod some more (cleanup)

go/types, types2: simplify missingMethod some more (cleanup)

Remove unnecessary assignments by using the same two variables
for methods consistently throughout.

Change-Id: I8b9bc8dbc8022b2472724d2a170f2635de3306e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/472915
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>