Remove coverage bots

See https://skia-review.googlesource.com/c/buildbot/+/156232

Bug: skia:
Change-Id: I731db6dd63d39e04b84a65dd7a18b7236d24a708
Reviewed-on: https://skia-review.googlesource.com/156365
Auto-Submit: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
9 files changed