Remove unused asACustomShader function from SkShader

I recentely removed the only call site of this so it should be
safe to delete now.

BUG=skia:

Change-Id: Id7fa94b9af5d73c4f7bb5e5a657d6fc6728923b5
Reviewed-on: https://skia-review.googlesource.com/7330
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
1 file changed