Drop QuickShare applet

This is dropped for 3 reasons:
- The concept of applications pushing open contents via SLC did not
materialise. Only dragging and dropping remains.

- The current implementation is broken. You cannot share more than once
between restarting plasmashell.

- The user experience of going to the panel to interact with contents
isnide an application seems confusing to me. There is ongoing work for a
"share portal" which is something I would happily push for.