Settings WebUI: Remove unnecessary shared CSS variables.

The following settings-* variables are mirroring existing cr-* variables.
and are not adding any value.

 --settings-box-row-padding -> --cr-section-padding
 --settings-box-row-indent -> --cr-section-indent-padding
 --settings-indent-width -> --cr-section-indent-width

Bug: None
Change-Id: Ib686d4bddfb8ad15d31edede06498feffe1ff462
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1568198
Commit-Queue: Dan Beam <dbeam@chromium.org>
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#651492}