Bug 1513035 - Remove text-align: center; from common.css buttons r=ewright,dao

This is already part of the button default styling, common.css does not need to set this again, especially since it breaks the menulist text alignment.

Differential Revision: https://phabricator.services.mozilla.com/D14119

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

toolkit/themes/shared/in-content/common.inc.css

@@ -176,7 +176,6 @@ html|*.numberbox-input::-moz-number-spin-down {
   background-color: var(--in-content-button-background);
   font-weight: 400;
   padding: 0 8px;
-  text-align: center;
   text-decoration: none;
   margin: 4px 8px;
   /* Ensure font-size isn't overridden by widget styling (e.g. in forms.css) */