Use Jekyll's markdownify filter instead of a custom plugin (#25319)

_plugins/markdown-block.rb

@@ -1,20 +0,0 @@
-module Jekyll
-  class MarkdownBlock < Liquid::Block
-    alias_method :render_block, :render
-
-    def initialize(tag_name, markup, tokens)
-      super
-    end
-
-    # Uses the default Jekyll markdown parser to
-    # parse the contents of this block
-    #
-    def render(context)
-      site = context.registers[:site]
-      converter = site.find_converter_instance(::Jekyll::Converters::Markdown)
-      converter.convert(render_block(context))
-    end
-  end
-end
-
-Liquid::Template.register_tag('markdown', Jekyll::MarkdownBlock)