Plugins are no longer an extensibility mechanism

With Flash no longer being supported, it shouldn't be listed as an example of the use of plugins. Furthermore, since #6003 aims to remove any complexity around plugins except for that which is needed for PDF viewers, it is clear that plugins in general are no longer an extensibility mechanism.

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@@ -413,9 +413,6 @@
    <li><p>Authors can embed raw data using the <code data-x="script">&lt;script type=""></code>
    mechanism with a custom type, for further handling by inline or server-side scripts.</p></li>
 
-   <li><p>Authors can create <span data-x="plugin">plugins</span> and invoke them using the
-   <code>embed</code> element. This is how Flash works.</p></li>
-
    <li><p>Authors can extend APIs using the JavaScript prototyping mechanism. This is widely used by
    script libraries, for instance.</p></li>