Obsolete osk-wayland script

The osk-wayland script enforces a fixed ordering (preferfing squeekboard over virtboard) and also makes it harder for downstreams as they need to figure out what is actually going on. By switching to update-alternatives (1) we can make things more flexible and get rid of the script. As we don't want to break users systems we make osk-wayland just play along for Byzantium by using update-alternatives with a low priority and we can finally drop it for crimson.

The only thing that we need to care about on the squeekboard side when importing (>= 1.20.0-2~) from Debian is that we add a Breaks: librem5-base-defaults (<< x) where x is the version that contains this MR.

Once we have that landed I'm happy to update virtboard to use update-alternatives too so it becomes actually selectable.

Note that the prerm fix in this also affects other things that were broken before (like conf file removal).

Marking as draft for the moment to not interfere with the audio and other changes.

Closes: #66 (closed)