Some RollbackManagerInternal cleanup (7/n)

As of ag/10689533, RollbackManagerInternal is a local service which will
not throw runtime exceptions. Let's remove unused try/catch blocks.

Bug: 150347230
Test: atest RollbackTest StagedRollbackTest
Change-Id: Ifde3e7dd2bd97abde27854a57b55b42b3e30799b