Remove QBluetoothSocketPrivate::socketDescriptor()

The function is not required as the public accessor function directly
returns the socket descriptor.

Change-Id: I6d64bec7b4f0542ce213dc90dcfe30c3885ed21d
Reviewed-by: Nedim Hadzic <nedimhadzija@gmail.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>