Normalize QObject::connect() statements

Some cases use the Q_PRIVATE_SLOT logic which does not require QObject
inheritance. Those cases were converted to lambda logic to avoid need for
QObject inheritance.

The Q_PRIVATE_SLOT macro was removed from qbluetoothdevicediscoveryagent.h.
This is not a BC problem because the macro expands to nothing. Only
moc recognizes the pattern.

Change-Id: Ic7cb4cde397f9b230b6fd0b4046e59e504583e58
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>