shill: cellular: simplify out-of-credits detection

shill now only uses the subscription state as an indicator for the
out-of-credits scenario. This CL simplifies the out-of-credits detection
by removing OutOfCreditsDetector and its subclasses.

BUG=b:112664666
TEST=Run unit tests.

Change-Id: Ia5ddf72bf2a11596c6fc9b76c9bf8e66c0a113e1
Reviewed-on: https://chromium-review.googlesource.com/1189429
Commit-Ready: Ben Chan <benchan@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Brian Norris <briannorris@chromium.org>