Remove htcacheclean references

[Oglopf]

There are references in the docs right now to htcacheclean needing to be ran, but it looks like this has been removed from the code base and so should also be removed from being mentioned in the docs.

Looks like there are a few mentions:

• https://osc.github.io/ood-documentation/latest/app-development/interactive/setup/enable-reverse-proxy.html?highlight=htcacheclean
• https://osc.github.io/ood-documentation/latest/installation/add-ssl.html?highlight=htcacheclean
• https://osc.github.io/ood-documentation/latest/installation/add-ldap.html?highlight=htcacheclean

The related ticket about this in the ondemand repo is here:

• Investigate the htcacheclean service ondemand#2273

Which points out the PR where this service was removed.

┆Issue is synchronized with this Asana task by Unito