cmd/compile/internal/gc: don't print "internal error" twice

The Fatalf mechanism already prints "compiler internal error:"
when reporting an error. There's no need to have "internal error"
in the error message passed to Fatalf calls. Removed them.

Fixes #28575.

Change-Id: I12b1bea37bc839780f257c27ef9e2005bf334925
Reviewed-on: https://go-review.googlesource.com/c/147287


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>