Remove MPRIS support

MPRIS is supported by Gwenview, but it's buggy and I think it's not really fixing any problems, instead bringing new ones. Notably, MPRIS will still be activated even when leaving a slideshow.

I then tried to make it so it didn't do that, but then realized this code isn't really doing anything notable - just making everything more complex.

• You can already the slideshow via the mouse, making stuff like the Media Player widget in Plasma make no sense to support.
• If you assign the "Start Slideshow" button to a keybind (which it isn't by default), all of the usual slideshow controls previously supported by MPRIS now works without it. You can go to the previous and next image, pause/play slideshow and all of that.

This is just a proposal, I personally never used this feature in Gwenview but I would like to hear if anyone would miss it. If so, I'll close this and focus on fixing the bugs in it instead.