Fix some compiler warnings.

As it turns out, we had an overloaded virtual from an earlier era,
with the extra parameter never used.  So cleaning that up was a
bonus to remove the compiler warning.

Change-Id: I780287f8a5d2b0a1ec84ec62c88ba50e051f372b
Reviewed-by: Jonas Rabbe <jonas.rabbe@nokia.com>

tests/auto/unit/qmediaserviceprovider/tst_qmediaserviceprovider.cpp

@@ -112,6 +112,7 @@ public:
 
     QList<QByteArray> devices(const QByteArray &service) const
     {
+        Q_UNUSED(service);
         QList<QByteArray> res;
         return res;
     }