[UDisksDeviceBackend] Remove pointless QDBusInterface
It's for checking whether the object exists but we shouldn't have come here in the first place if it isn't.
In any case, it does a blocking introspection call, and the connects that come thereafter don't really hurt.
On my system Solid spends a good 50ms (48 devices, lots of snap mounts, etc) on Dolphin startup in this.