Delete legacy image generator flag from bzl

NOTRY=TRUE

Bug: skia:
Change-Id: Icad74e486d957d10cb3e1091525991cd910626b7
Reviewed-on: https://skia-review.googlesource.com/16717
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
1 file changed