cmd/compile/internal/gc: various minor cleanups

cmd/compile/internal/gc: various minor cleanups

Two funcs and a field were unused. Remove them.

A few statements could be made simpler.

importsym's pos parameter was unused, so remove it.

Finally, don't use printf-like funcs with constant strings that have no
formatting directives.

Change-Id: I415452249bf2168aa353ac4f3643dfc03017ee53
Reviewed-on: https://go-review.googlesource.com/117699
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>