test/syntax: remove interface.go

The exact same test case covered by this file is also in
fixedbugs/bug121.go. No need for duplication.

Also, the actual syntax error tested (multiple method names
with a single signature) is an unlikely syntax error, and
only here for historical reasons (in the very beginning, this
was actually possible to write). Now, virtually nobody is making
this mistake.

Change-Id: I9d68e0aee2a63025f44e6338647f8250ecc3077a
Reviewed-on: https://go-review.googlesource.com/c/go/+/307789


Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>