FIX: Spoiler logic should live inside of spoiler plugin

This commit removes logic about spoilers because it should live inside
of the discourse-spoiler-alert plugin.

This PR:

https://github.com/discourse/discourse-spoiler-alert/pull/38

also completely removes spoilers from excerpts in order to keep them
from leaking in topic previews and notifications.

lib/excerpt_parser.rb

@@ -123,11 +123,6 @@ class ExcerptParser < Nokogiri::XML::SAX::Document
         @current_length = 0
         @start_excerpt = true
       end
-      # Preserve spoilers
-      if attributes.include?(["class", "spoiler"])
-        include_tag("span", attributes)
-        @in_spoiler = true
-      end
 
     when "details"
       @detail_contents = +"" if @in_details_depth == 0