Hide 'View plan' button in data usage settings

There are currently no callers of the setSubscriptionPlan() API that
have a MDP app set up. Remove the button when SubscriptionPlans are
defined, since it does nothing and causes confusion for users.

Test: manually verify button does not exist when a plan is defined
Test: make RunSettingsRoboTests
Bug: 169806315
Change-Id: Ieda020bfa692e9465d6297e50cf338deff91a8a9