Revert "[RN] Use rounded avatars in the film strip"

This reverts commit 739298c782.

react/features/base/participants/components/styles.js

      * Avatar style.
      */
     avatar: {
-        alignSelf: 'center',
-
-        // FIXME I don't understand how a 100 border radius of a 50x50 square
-        // results in a circle.
-        borderRadius: 100,
         flex: 1,
-        height: 50,
-        width: 50
+        width: '100%'
     },
 
     /**