Remove the old StandAloneAppCorona

Early Plasma 5 has an "app mode" to be started as a normal app (with
normal windows rather than desktops and panels)

This was ever used only by Plasma Media Center (comletely different from
Plasma Bigscreen) which is discontinued since years, and this has never
been used in years, being just dead code and potentially a sounce of
bugs at this point.

Remove it (if we will really think it was a good idea can be re-added
down the road at any point) to lean up plasma 6 a bit