I realized `--redownload` is not a good name, because it does not
necessarily redownloads gems. It only forces reinstallation even if gem
is already installed.

So I believe `--force` is actually a better name and the introduction of
`--force` was a misunderstanding of what the `--force` flag did at the
time.

Let's cancel the deprecation of `--force`.

For now the `--redownload` alias is left around until we decide what to
do with it.