Delete legacy image generator API flag

Bug: skia:6620
Change-Id: I7ecdb5b162d8d468d91d4c8dbe276fabe349dc15
Reviewed-on: https://skia-review.googlesource.com/17215
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
2 files changed