Remove SK_TREAT_COLINEAR_DIAGONAL_POINTS_AS_CONCAVE

This was rebaselined and removed from Chrome long ago.
Planning to fix more bugs in the Convexicator, but want
to get this out of the way first.

Change-Id: I4f299d56a81be509a861d1e9c9f0e48a47170096
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/209322
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>