cmd/compile: simplify noding for struct embedding

cmd/compile: simplify noding for struct embedding

Since golang.org/cl/31670, we've stopped using the 'embedded' function
for handling struct embeddings within package export data. Now the
only remaining use is for Go source files, which allows for some
substantial simplifications:

1. CenterDot never appears within Go source files, so that logic can
simply be removed.

2. The field name will always be declared in the local package.

Passes toolstash-check.

Change-Id: I59505f62824206dd5de0782918f98fbef6e93224
Reviewed-on: https://go-review.googlesource.com/58790
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>