Remove flag: explicit_kill_from_system_server

Early removal of flag to land it on AOSP faster.

Bug: 343394964
Bug: 339548431
Fix: 339548431
Test: atest BluetoothInstrumentationTests
Flag: com.android.bluetooth.flags.explicit_kill_from_system_server
Change-Id: Ica4e3cfff4761d7a5abc42d3c4f542651e780f8a