SkBitSet: cleanup. Remove unused fn, more general exporter.

Change-Id: I090a20decf30631f2464a820c6a056a81b6cc1a3
Reviewed-on: https://skia-review.googlesource.com/c/163784
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
5 files changed