Delete deprecated SkDropShadowImageFilter header

Bug: skia:11230
Change-Id: I9da638b5e1de50aee9a805547f51e013e55747b4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/368598
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
6 files changed