fix(css): do not display avatar on top of the messages

Previously the z-index was introduced in order to have the avatar
visible on top of the black video element, but now we're always hiding
the video element when the avatar is displayed, so it's no longer
required.

css/_videolayout_default.scss

     margin: auto;
     position: relative;
     border-radius: 100px;
-    z-index: 3;
     visibility: inherit;
     background-color: #000000;
 }