Remove WifiManager.getTxPacketCount

This is no longer called by CTS, and has no other
clients.

This also removes the dependency on
WifiNl80211Manager.getTxPacketCounters(), so it
will no longer need test coverage.

Bug: 150978929
Test: atest FrameworksWifiApiTests
Change-Id: Ia64a6b2f5009c021ed7dd66c24e854996995778a