Dev: admin RTL, removed unused files, added some comments

Many RTL CSS files in admin themes had been generated before the refactorisation
of CSS (creation of lime-admin-colors.css && lime-admin-common.css).
So it was having irrelevant files
(like lime-admin-misc-rtl.css or lime-admin-rtl.css).

Also, the script to generate the CSS was applied to all css files,
inclued for irrelevant files
(like lime-admin-colors-rtl.css, which contains only colors, or fonts-rtl.css
which contains only font definition).

The comments added to config files try to make the process easier to understand
for external devs.