QBluetoothSocket - deduplicate the code (macOS)

Implement the proper interface from QBluetoothSocketBasePrivate,
remove a dummy base class. Remove all public API from
qbluetoothsocket_osx.mm and re-use the code in qbluetoothsocket.cpp.
The code generally is the same, a bit of re-hashin/deletion.

Task-number: QTBUG-75348
Change-Id: I0034dfd283daf9d51775d8f9551b85d2d436aa85
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>