Removed md-file CSS class

The class has always been used in combo with the simple .md.
Updated all the usages to use only the .md instead of the
whole couple.

app/assets/stylesheets/framework/typography.scss

@@ -59,7 +59,7 @@
     max-width: 100%;
   }
 
-  &:not(.md-file) img:not(.emoji) {
+  &:not(.md) img:not(.emoji) {
     border: 1px solid $white-normal;
     padding: 5px;
     margin: 5px 0;

app/views/projects/_wiki.html.haml

 - if @wiki_home.present?
   %div{ class: container_class }
-    .md.md-file.prepend-top-default.append-bottom-default
+    .md.prepend-top-default.append-bottom-default
       = render_wiki_content(@wiki_home)
 - else
   - can_create_wiki = can?(current_user, :create_wiki, @project)

app/views/projects/blob/preview.html.haml

 - if markup?(@blob.name)
-  .file-content.md.md-file
+  .file-content.md
     = markup(@blob.name, @content)
 - else
   .diff-file

app/views/projects/blob/viewers/_markup.html.haml

 - blob = viewer.blob
 - context = blob.respond_to?(:rendered_markup) ? { rendered: blob.rendered_markup } : {}
-.file-content.md.md-file
+.file-content.md
   = markup(blob.name, blob.data, context)

app/views/projects/wikis/show.html.haml

@@ -26,7 +26,7 @@
     = (s_("WikiHistoricalPage|You can view the %{most_recent_link} or browse the %{history_link}.") % { most_recent_link: most_recent_link, history_link: history_link }).html_safe
 
 .prepend-top-default.append-bottom-default
-  .md.md-file{ data: { qa_selector: 'wiki_page_content' } }
+  .md{ data: { qa_selector: 'wiki_page_content' } }
     = render_wiki_content(@page)
 
 = render 'sidebar'

app/views/search/results/_snippet_blob.html.haml

@@ -23,7 +23,7 @@
         %i.fa.fa-file
         %strong= snippet.file_name
     - if markup?(snippet.file_name)
-      .file-content.md.md-file
+      .file-content.md
         - snippet_chunks.each do |chunk|
           - unless chunk[:data].empty?
             = markup(snippet.file_name, chunk[:data])