xmlui: Clean up superfluous font declarations

Unneeded font-family declarations in base.css and style.css:

find modules/xmlui/src/ -iname "base.css" -exec sed -i "/font-family:/d" {} \;
find modules/xmlui/src/ -iname "style.css" -exec sed -i '16,${/font-family/d}' {} \;

This makes the fonts more standard and predictable, and also saves
parsing time.