[Glitch] Fix transparent avatars showing underlying elements in avatar stack and profile

Port 79227d153b to glitch-soc

app/javascript/flavours/glitch/styles/_mixins.scss

@@ -1,6 +1,5 @@
 @mixin avatar-radius() {
   border-radius: $ui-avatar-border-size;
-  background: transparent no-repeat;
   background-position: 50%;
   background-clip: padding-box;
 }