Improvements for the Tor Browser-specific parts of the updater changes

At a certain point, we moved the updater commit to Base Browser and extracted the Tor Browser-specific parts to another commit.

I think this commit could see some improvements.

directories_to_remove and extra_files_to_remove

We have some hunks for them, but nothing uses them. I think they were used up until 68 for HTTPS-E. 78 didn't use them already.

directories_to_remove is not even defined in tools/update-packaging/make_full_update.sh.

So, I think we could delete these parts.

add_if_update

For the fontconfig.conf problem of 12.5aX (after I moved it from TorBrowser/Data, #41776 (closed)), we added/restore a new functionality: add_if_update.

It could go to a commit on its own, since we'd like to remove it at a certain point.

At least I could add a few more comments also to tools/update-packaging/make_incremental_update.sh.