remove old docs

  - c++11.md isn't really needed any more
  - flatten.md makes reference to code that doesn't exist
  - simd.md describes a whole system that doesn't exist
  - color.md explains a model that's fundamentally different
    to how color correction works in Skia now

Change-Id: Ib09678190d04a0dcf44eadf13c617d488307332b
Reviewed-on: https://skia-review.googlesource.com/c/178080
Reviewed-by: Heather Miller <hcm@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
12 files changed