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 FrameworksWifiTests
Change-Id: I5497d9000d05b53bff020f6643e1ef61aee4f763
6 files changed