remove to_2dot2 and from_2dot2

remove to_2dot2 and from_2dot2

The parametric_{r,g,b} stages are just as good now;
under the hood it's all going through approx_powf.

Change-Id: If7f3ae1e24fcee2ddb201c1d66ce1dd64820c89a
Reviewed-on: https://skia-review.googlesource.com/14320
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>