cmd/compile/internal/gc: remove unused pkgByPath type

cmd/compile/internal/gc: remove unused pkgByPath type

pkgByPath was added in d78c84c4 to eliminate the differences between the
export formats around the time of Go 1.7.

The last remnants of the textual export format was removed by Josh in
39850 making the pkgByPath sorting type unused.

Change-Id: I168816d6401f45119475a4fe5ada00d9ce571a9e
Reviewed-on: https://go-review.googlesource.com/40050
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>