Bug 31542: Home page links correct font-family

Bug 31542: Home page links correct font-family

In koha-tmpl/intranet-tmpl/prog/css/mainpage.css the rule of
font-family for big links is not complete :
  font-family: verdana, arial;
It is missing sans-serif in case computer has not those fonts.
On my computer I dont have them so I see a serif font.

We can remove this rule, inherited rule is serif fonts.

Test plan :
1) Use a computer without Arial nor Verdana fonts (basically a Linux)
2) Look at home page big links : Circulation, Patrons ...
   => Without patch they have serif font
   => With patch they have sans-serif font

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>