Correct retina icons on a forked debugger plugin

When a custom debugger plugin is used, order of the css compilation changes and causing wrong display on a forked debugger with retina display.
Removing this important flag solves the issue.
I have not seen any side effects of removing this important flag

plugins/c9.ide.layout.classic/less/c9-menu-btn.less

@@ -65,7 +65,7 @@
     display: none;
 }
 
-.c9-menu-btn.nosize .icon { background-size: auto !important; }
+.c9-menu-btn.nosize .icon { background-size: auto; }
 
 .c9-menu-btn.preferences{
     padding: @preference-menu-button-padding !important;
@@ -116,4 +116,4 @@
 .c9-menu-btn.c9btn.offline{
     color: #d22a3f;
     text-shadow: 0 1px black;
-}
+}