Remove some unused l10n strings

[ddernon]

First commit addresses this Mantis ticket https://freenet.mantishub.io/view.php?id=6742

Then I looked for more unused strings.

The 3rd commit removes strings related to removed default bookmarks.

The other 2 remove strings in various areas. A bit hard to make sure a string is unused without having any idea where it used to be used in the first place though, since some strings have pretty short IDs and may be loaded with something else than the usual l10n() or NodeL10n.getBase().getString(). I'm currently running a build with all these removed and trying to see if I notice anything missing.

[ddernon]

I've been using this build for a while now and didn't notice any missing string 👀

[ArneBab]

Thank you for your cleanup! Did you check whether they could be used in plugins?

[ddernon]

I was unaware that those strings could be used from plugins... Is there a way to make sure no plugin use them? Or should I just pick a list of plugins, maybe this one, and check them and ignore what could happen with plugins that aren't on this list?