Bug 1519083 - Remove unused .devtools-menulist selector r=ntim

Remove unused .devtools-menulist selector

Differential Revision: https://phabricator.services.mozilla.com/D17956

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

devtools/client/themes/common.css

@@ -285,7 +285,6 @@ checkbox:-moz-focusring {
 }
 
 /* Toolbar buttons */
-.devtools-menulist,
 .devtools-toolbarbutton,
 .devtools-button {
   -moz-appearance: none;