Remove spellcheck workflow

Unfortunately, the separate spellcheck GHA workflow does not really work
with homu, if we would like to enforce the invariant that PR CI is a
subset of Full CI (modulo carve outs).

This is not prejudice against a reland of a `typos`-based spellcheck, it
just probably has to go through the "usual" CI flow with bootstrap, so
that it can work with homu.