Remove plugins from Earmark config options (#1113)

With #1109 the Earmark
dependency was set to `~> 1.4`

Earmark 1.4 no longer supports plugins see (https://github.com/pragdave/earmark/blob/master/RELEASE.md#140-20190905)
so setting this option through `ex_doc` is not possible anymore.

lib/ex_doc/markdown/earmark.ex

@@ -24,10 +24,6 @@ defmodule ExDoc.Markdown.Earmark do
     * `:smartypants` - boolean. Turns on smartypants processing, so quotes become curly,
       two or three hyphens become en and em dashes, and so on. False by default
 
-    * `:plugins` - map of strings to modules. Register custom plugins to be used with
-      Earmark. See [Plugins](http://github.com/pragdave/earmark#plugins) for details on
-      how to write custom plugins.
-
     * `:highlight_tag` - the tag used for highlighting code, defaults to "span"
 
   """
@@ -39,8 +35,7 @@ defmodule ExDoc.Markdown.Earmark do
         file: Keyword.get(opts, :file),
         breaks: Keyword.get(opts, :breaks, false),
         smartypants: Keyword.get(opts, :smartypants, false),
-        pure_links: true,
-        plugins: Keyword.get(opts, :plugins, %{})
+        pure_links: true
       )
 
     text