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>

src/go/internal/gccgoimporter/gccgoinstallation_test.go

@@ -62,8 +62,6 @@ var importablePackages = [...]string{
 	"encoding/pem",
 	"encoding/xml",
 	"errors",
-	"exp/proxy",
-	"exp/terminal",
 	"expvar",
 	"flag",
 	"fmt",
@@ -114,8 +112,6 @@ var importablePackages = [...]string{
 	"net/smtp",
 	"net/textproto",
 	"net/url",
-	"old/regexp",
-	"old/template",
 	"os/exec",
 	"os",
 	"os/signal",