shill: cellular: Delete MobileOperator.

This is a half-completed class that we never ended up using. It has been
superseded by |MobileOpertorInfo|.

BUG=chromium:371630
TEST=Run shill_unittests.

Change-Id: I4a13bd2676246836a47bac1ba3cabac7d0a0dccb
Reviewed-on: https://chromium-review.googlesource.com/201102
Reviewed-by: Thieu Le <thieule@chromium.org>
Commit-Queue: Prathmesh Prabhu <pprabhu@chromium.org>
Tested-by: Prathmesh Prabhu <pprabhu@chromium.org>