QBluetoothDeviceDiscoveryAgent - cleanup (OS X/iOS)

Replace handwritten 'signatures' with Q_FUNC_INFO, remove spurious
debug messages. Get rid of 'transient delegate' trick.

Change-Id: I2c490271baba7247af11df128964b61d0255ef95
Reviewed-by: Alex Blasche <alexander.blasche@theqtcompany.com>