Simplify unregister logic for Connectivity Diagnostics callbacks.

ConnectivityService is updated to simplify the logic for unregistering
ConnectivityDiagnosticsCallback instances. This change removes the given
callback from ConnectivityService's data structure. If the callback was
not registered with ConnectivityService, it is logged and the function
exits; else, the unregister() operation continues.

Bug: 150867635
Test: atest FrameworksNetTests
Change-Id: I9096969a1bf33da72b117f5bbc88257df805e688
1 file changed