[skottie] Drop motion blur support

The current approach is slow, yields questionable quality, and requires
some hacks to mutate the scene graph during rendering.

AFAIK nobody is using motion blur with Skottie.

Change-Id: I2b55201e902f8c7d1b1936e0e21515bb84573277
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/945118
Reviewed-by: Brandon Wilson <bwils@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>
8 files changed