Fix race in removeDeadProvider.

Bug: 5202824

It's possible for removeDeadProvider to be called after the provider
has already been removed from the provider map due to a race between
binderDied and removing the provider.

Deleted removeDeadProviderLocked; it was dead code.

Change-Id: Iecdc68703225e7ac171746e63f1b3141c6f2ce4c