Remove SK_IGNORE_QUAD_RR_CORNERS_OPT-related code.

Now that the flag is no longer used in Chromium
(https://codereview.chromium.org/426593006/), we can remove it.

R=bsalomon@google.com, reed@google.com, robertphillips@google.com

Review URL: https://codereview.chromium.org/430473012