Make osk-wayland obsolete

squeekboard 1.20.0-2 from Debian will use update-alternatives to manage
the /u/s/a/sm.puri.OSK0.desktop finally allowing us to obsolete
osk-wayland here.

We do so by making sm.puri.OSK0-legacy.desktop use update-alternatives
too and giving it a lower priority than squeekboards.

As older squeekboard in PureOS Byzantium doesn't worry about
sm.puri.OSK0 at all so an update of librem5-base-defaults won't change
anything for the user.

We can drop sm.puri.OSK0-legacy and osk-wayland for Crimson