Bug 23078: (follow-up) Update self checkout help page

Bug 23078: (follow-up) Update self checkout help page

This patch takes care of some preference instances I missed in the self
checkout module's help page. It also removes some obsolete lines from
Auth.pm.

To test, apply the patch and re-test based on the previous test plan.
Also test in the self-checkout module by logging into self checkout and
clicking the "Help" link in the upper right of the screen.

The settings of the following preferences should be shown correctly:

 - OpacFavicon
 - OPACUserCSS

Signed-off-by: frederik chenier <frederik.chenier@inlibro.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
(cherry picked from commit b5588958981a24c25536afff0b2091506bf7f92b)
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>