Chromiun no longer use set_sources_assignment_filter() anywhere in the
build, so these are no longer needed.

Bug: chromium:1018739
Change-Id: Ibaf8c11c29fc0bbd63afded3e4a52bd5e07a8283
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2456588Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>