Revert "UX: Style video elements, show descriptions (#10040)" (#10258)

This reverts commit 7d289a4f3e.

Now that 36bad0c31f is in and we have video previews on all platforms, the commit that's being reverted is no longer needed. In the worst case scenario, the video description is clipped under the video poster if the video aspect ratio is other than 16:9. This commit removes descriptions and the custom style for the video elements.

# Conflicts:
#	app/assets/javascripts/pretty-text/addon/engines/discourse-markdown-it.js
#	test/javascripts/lib/pretty-text-test.js

test/javascripts/lib/pretty-text-test.js

@@ -1029,7 +1029,6 @@ QUnit.test("video", assert => {
   assert.cooked(
     "![baby shark|video](upload://eyPnj7UzkU0AkGkx2dx8G4YM1Jx.mp4)",
     `<p><div class="video-container">
-    <p class="video-description">baby shark</p>
     <video width="100%" height="100%" preload="metadata" controls>
       <source src="/404" data-orig-src="upload://eyPnj7UzkU0AkGkx2dx8G4YM1Jx.mp4">
       <a href="/404">/404</a>
@@ -1056,7 +1055,6 @@ QUnit.test("video - mapped url - secure media enabled", assert => {
       lookupUploadUrls: lookupUploadUrls
     },
     `<p><div class="video-container">
-    <p class="video-description">baby shark</p>
     <video width="100%" height="100%" preload="metadata" controls>
       <source src="/secure-media-uploads/original/3X/c/b/test.mp4">
       <a href="/secure-media-uploads/original/3X/c/b/test.mp4">/secure-media-uploads/original/3X/c/b/test.mp4</a>