pad.css: group togheter three very similar rules.
pad.css: group togheter three very similar rules.

a) these rules:
  [class^="icon-"]:before
  [class*=" icon-"]:before

b) were the same as this one:
  [data-icon]:before

except the rules in b) had a "content: attr(data-icon)" rule, too.

This commit groups all of them together, and gets rid of the "attr(data-icon)".

The commit that introduced these rules in the first place, and that are now
partially reverted, was 9aea689438d364cfd89a476d06729b3dd1195b8e (move tiny bit
of font awesome we actually use into pad.css) from 2014-11-19.

Preparatory work for introducing colibris skin

storytouch/etherpad-lite,joequant/etherpad-lite,joequant/etherpad-lite,ether/etherpad-lite,aptivate/etherpad-lite,storytouch/etherpad-lite,joequant/etherpad-lite,ether/etherpad-lite,aptivate/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,cloudfoundry-community/etherpad-lite-cf,aptivate/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,cloudfoundry-community/etherpad-lite-cf,joequant/etherpad-lite,ether/etherpad-lite,aptivate/etherpad-lite,storytouch/etherpad-lite