Remove references to `--dev-only` install option

According to
https://github.com/python-poetry/poetry/issues/4864#issuecomment-1002465303:
> The `--dev-only` option was never available in a release, but was already
> available in the master branch. Because it's a feature we will probably not
> backport this to the 1.1 branch.

Given that the option was only ever present when installing Poetry from master
and was never available as part of a release, I think it's clearer to just
remove all references to it rather than include a deprecation notice that would
only be relevant to a tiny fraction of Poetry users.