Update gn_to_bp

https://skia-review.googlesource.com/c/skia/+/1013977 landed some
changes which were intended for an earlier version of the CL (with :pdf
and :pathops removed from :skia deps).

This CL removes all the added gunk, and just adds :pathops to the
dependency cutoff list for app sources.

Change-Id: Idca95713e7cadd7c45db13d125431dc7d1f4c60a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1015556
Commit-Queue: Florin Malita <fmalita@google.com>
Reviewed-by: Noelle Scobie <nscobie@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>