a11y: Removes duplicate header from wiki pages

Removes the SR-only header and replaces the h2 page title
with an h1 page title (maintaining the same style) which
keeps a duplicate header from being read by SR.

fixes CNVS-16686

Test Plan:
  - Go to a wiki page
  - Screenreader should only have the page title header
    detected once, as a H1.

Change-Id: Ie4f4bb1bd45b50ae62aeb91e4df50392c0e73cec
Reviewed-on: https://gerrit.instructure.com/43854
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Dan Minkevitch <dan@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Dan Minkevitch <dan@instructure.com>

app/views/jst/wiki/WikiPage.handlebars

@@ -67,7 +67,7 @@
 {{/if}}
 
 <div class="show-content user_content clearfix">
-  <h2 class="page-title">{{title}}</h2>
+  <h1 class="page-title">{{title}}</h1>
   {{#if locked_for_user}}
     <div class="locked-alert alert alert-warning">
       {{#if lock_info.context_module.prerequisites}}

app/views/wiki_pages/show_page.html.erb

@@ -2,5 +2,4 @@
   content_for :page_title, join_title(@page.title.to_s, @context.name)
   js_bundle :wiki_page_show
 %>
-<h1 class="screenreader-only"><%= @page.title %></h1>
 <div id="wiki_page_show"></div>