go/internal/gccgo: remove unused test file

Follow-up on https://go-review.googlesource.com/c/go/+/137857/4
which didn't remove this test file after it was removed from the
list of importer tests in importer_test.go.

Change-Id: Ib89cb3a6d976115da42c33443529ea27bd1ce838
Reviewed-on: https://go-review.googlesource.com/137975
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/go/internal/gccgoimporter/testdata/alias.gox

-v1;
-package alias;
-pkgpath alias;
-type <type 115 "I1" <type 116 interface { M1 (? <type 117 "IntAlias2" = <type 118 "IntAlias" = <type 119 "Int" <type -11>>>>) < type 114>; M2 () <type 1>; }>>;