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
Merged-In: I9096969a1bf33da72b117f5bbc88257df805e688
(cherry picked from commit f0473139)