Remove Exception in getRegisteredExperimentIds

This check was not useful since service can only be null if system
server is not running or the app does not have permissions to access
statsd. This check is not done anywhere else, so removing it for
consistency.

Bug: 150645647
Test: atest StatsdHostTestCases
Change-Id: I6465697f0157c5a9583025979c55cb2e3dc1ef84