cellular: Remove obsolete and unused mock_modem.py

BUG=chromium:404003
TEST=Trybot run on paladin, release, and chromiumos-sdk builders.

Change-Id: I94bc532985ede8aedf7ee228b19bf67d4b59b1fe
Reviewed-on: https://chromium-review.googlesource.com/212811
Tested-by: Ben Chan <benchan@chromium.org>
Reviewed-by: Prathmesh Prabhu <pprabhu@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
1 file changed