feat(plugins): allow plugin deregistration from videojs (#5273)

This exposes `deregisterPlugin` on the videojs alongside `registerPlugin`.

src/js/video.js

@@ -464,6 +464,20 @@ videojs.bind = Fn.bind;
  */
 videojs.registerPlugin = Plugin.registerPlugin;
 
+/**
+ * Deregister a Video.js plugin.
+ *
+ * @borrows plugin:deregisterPlugin as videojs.deregisterPlugin
+ * @method deregisterPlugin
+ *
+ * @param  {string} name
+ *         The name of the plugin to be deregistered. Must be a string and
+ *         must match an existing plugin or a method on the `Player`
+ *         prototype.
+ *
+ */
+videojs.deregisterPlugin = Plugin.deregisterPlugin;
+
 /**
  * Deprecated method to register a plugin with Video.js
  *