Revert of add isRect() check to AAClip, to detect if a soft-clip is really just an...

Revert of add isRect() check to AAClip, to detect if a soft-clip is really just an irect (https://codereview.chromium.org/445233006/)

Reason for revert:
new code asserts on some pictures

Original issue's description:
> add isRect() check to AAClip, to detect if a soft-clip is really just an irect
>
> BUG=skia:
>
> Committed: https://skia.googlesource.com/skia/+/592cb8d552556b1e922887d506d00b64bc5d0547

R=bsalomon@google.com, humper@google.com
TBR=bsalomon@google.com, humper@google.com
NOTREECHECKS=true
NOTRY=true
BUG=skia:

Author: reed@google.com

Review URL: https://codereview.chromium.org/452533002