go/types, types2: remove unused code in lookupFieldOrMethod

The underlying type of a type parameter is an interface,
so we don't need a special case for type parameters anymore.
Simply share the (identical) code for interfaces.

Adjust code in types.NewMethodSet accordingly.

No functional difference.
Preparation for fix of issues below.

For #50233.
For #50417.

Change-Id: Ib2deadd12f89e6918dec224b4ce35583001c3101
Reviewed-on: https://go-review.googlesource.com/c/go/+/375514


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>