remove unused maskfilter helpers: Combine, Compose

Change-Id: I8c6bf2c3539905c9c7bc9c29454322dbf944fe96
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/287823
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
4 files changed