Change text extraction in `PlainTextFormatter` to be faster (#26727)

app/lib/plain_text_formatter.rb

 # frozen_string_literal: true
 
 class PlainTextFormatter
-  include ActionView::Helpers::TextHelper
-
   NEWLINE_TAGS_RE = %r{(<br />|<br>|</p>)+}
 
   attr_reader :text, :local
@@ -18,7 +16,10 @@ class PlainTextFormatter
     if local?
       text
     else
-      html_entities.decode(strip_tags(insert_newlines)).chomp
+      node = Nokogiri::HTML.fragment(insert_newlines)
+      # Elements that are entirely removed with our Sanitize config
+      node.xpath('.//iframe|.//math|.//noembed|.//noframes|.//noscript|.//plaintext|.//script|.//style|.//svg|.//xmp').remove
+      node.text.chomp
     end
   end
 
@@ -27,8 +28,4 @@ class PlainTextFormatter
   def insert_newlines
     text.gsub(NEWLINE_TAGS_RE) { |match| "#{match}\n" }
   end
-
-  def html_entities
-    HTMLEntities.new
-  end
 end