fix(premeeting) drop gradient

It looks particularly odd, specially when any virtual background is activated.

css/_premeeting-screens.scss

 /**
  * Shared style for full screen local track based dialogs/modals.
  */
- .premeeting-screen,
-  .preview-overlay {
+ .premeeting-screen {
      position: absolute;
      left: 0;
      right: 0;
         }
     }
 
-    .preview-overlay {
-        background-image: linear-gradient(transparent, black);
-        z-index: $toolbarZ + 1;
-    }
-
     .content {
         align-items: center;
         display: flex;

react/features/base/premeeting/components/web/PreMeetingScreen.js

                 <Preview
                     videoMuted = { videoMuted }
                     videoTrack = { videoTrack } />
-                {!videoMuted && <div className = 'preview-overlay' />}
                 <div className = 'content'>
                     {showAvatar && videoMuted && (
                         <Avatar