Bug 1496193: Remove unused datepicker[type=grid] selector r=bgrins

Removed unused datepicker[type=grid] selector

Differential Revision: https://phabricator.services.mozilla.com/D24643

--HG--
extra : moz-landing-system : lando

toolkit/content/xul.css

@@ -73,7 +73,6 @@ xbl|children {
 
 button,
 checkbox,
-datepicker[type="grid"],
 menulist,
 radiogroup,
 tree,