Hygiene: Remove BogusMobileContext
The BogusMobileContext and MobileContext::setInstanceForTesting method
were used to create an implicit test that MobileContext::singleton
wasn't called throughout the MobileContextTest test suite.
Since MobileContext::singleton, ::setInstanceForTesting, and
::resetServiceForTesting were deprecated in 535f18b1 and will be removed
as part of T143189, it's no longer worth maintaining BogusMobileContext