osk-wayland: Don't use squeekboard-restyled

Squeekboard as of 1.15.0 is clever enough to do the right thing when run
with Phosh and not using squeekboard-restyled unbreaks HighContrast.

We keep this layer of indirection for the moment to avoid yet another
phosh release and will switch to what Debian does for its session
(using sm.puri.Squeekboard directly) with the next or next next phosh
release).