main/postmarketos-ui-phosh: drop osk.sh

Since c49d84f4ab1856156d26d59f5c88387776990d24 in Alpine's aports, the sm.puri.OSK0.desktop service no longer tries to execute /usr/bin/osk-wayland and instead just executes /usr/bin/squeekboard directly, thus avoiding this needless indirection. virtboard hasn't existed in postmarketOS for years at this point and squeekboard is a hard dependency of phosh, so it's always going to be present (thus making this check unnecessary) and the fallback if it isn't wouldn't work anyway.