go/internal/gccgoimporter: remove old and exp gccgo packages in test

The old and exp packages listed in gccgoinstallation_test.go have been
removed from gccgo. Remove them from the test.

Fixes #20932

Change-Id: I04a5148e18dccef332904b836c42098b55f2516c
Reviewed-on: https://go-review.googlesource.com/91656


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