remove dead code around SK_SUPPORT_LEGACY_CANVAS_READPIXELS

Bug: skia:6513
Change-Id: I8e4e0ffb371ae5b1af972110339e57d491ad9ff1
Reviewed-on: https://skia-review.googlesource.com/13779
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
3 files changed