[plugins] drop the RemoveSlashAtEnd() from plugin options

For some weird reason (from ages ago - far enough back that I got bored looking further) we remove the slash at the end of plugin _options_ even though there's no need to do so: Both URIUtils::AddSlashAtEnd, HasSlashAtEnd etc. correctly ignore protocol options when adding or checking for slashes.

Needs extensive testing.

@MartijnKaijser perhaps you could test?