Remove pregenerated manpages from the repo

We don't need them in the repo, it's sufficient if we pregenerate them
while preparing the tarball. That way we don't have overhead while
modifying them but they are still available for installations without
Sphinx.

I assume that this will make rebases and cherry-picks across branches
easier, with less trial and error churn required in the CI.
65 jobs for !6520 with pspacek/make-manin-on-dist in 3 minutes and 55 seconds (queued for 2 seconds)
merge request