cmd/compile: stop treating interface methods like actual functions

Interface methods can't have function bodies, so there's no need to
process their parameter lists as variable declarations. The only
possible reason would be to check for duplicate parameter names and/or
invalid types, but we do that anyway, and have regression tests for it
(test/funcdup.go).

Change-Id: Iedb15335467caa5d872dbab829bf32ab8cf6204d
Reviewed-on: https://go-review.googlesource.com/31430


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>