remove unused mode parameter from SkMergeImageFilter

Bug: skia:
Change-Id: Iaa46aaef130a337987c3528685f59c56387d4a7d
Reviewed-on: https://skia-review.googlesource.com/20210
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
12 files changed