gtk3: Remove unnecessary extend from _budgie.scss

This didn't seem to have any effect with budgie-desktop-10.5.1, and
only increased the compiled CSS size by nearly 19kB.

Author: jnsh

common/gtk-3.0/3.24/sass/_budgie.scss

@@ -307,8 +307,6 @@ $z-depth-4: rgba(0, 0, 0, 0.34);
   }
 
   button {
-    @extend row, .activatable;
-
     min-height: 32px;
     padding: 0 8px;
     border-radius: 0;