cmd/compile/internal/typecheck: remove unused -G=0 code

The typechecking code for dealing with dot imports and redeclaration
errors can be removed, as these will now always be caught by types2
instead. Even when running the typecheck on internally constructed IR,
we'll never introduce new imports or redeclare identifiers.

Also, Func.Shortname (and typecheck.addmethod) was only used by the
-G=0 frontend. The new types2-based frontends directly associate
methods with their receiver type during IR construction.

Change-Id: I6578a448412141c87a0a53a6566639d9c00eeed7
Reviewed-on: https://go-review.googlesource.com/c/go/+/388537


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>