wiki_pages/show: fix <meta> description tag.

Use an excerpt of the first paragraph rather than the entire page.

app/presenters/wiki_page_presenter.rb

@@ -5,10 +5,6 @@ class WikiPagePresenter
     @wiki_page = wiki_page
   end
 
-  def excerpt
-    wiki_page.body
-  end
-
   # Produce a formatted page that shows the difference between two versions of a page.
   def diff(other_version)
     pattern = Regexp.new('(?:<.+?>)|(?:[0-9_A-Za-z\x80-\xff]+[\x09\x20]?)|(?:[ \t]+)|(?:\r?\n)|(?:.+?)')