MediaDeviceCache: work-around Solid UDisks2 bug by not calling allDevices()

Kudos go to Alex Fiestas for extensive debugging and finding out the
root of the problem.

BUGFIXES:
 * Work-around Solid UDisks2 backend bug that caused USB Mass Storage
   devices and iPods not being recognized when connected before Amarok
   was started. Also fixes a bug where blank devices would appear in
   Play Media dialog. (BR 322980)

We don't need to call Solid::Device::allDevices() at all because that
was only needed for Apple iPhone support for the *old* iPod collection
that was replaced in Amarok 2.6.

BUG: 322980
CCBUG: 317485
FIXED-IN: 2.8