Remove superfluous if/else branching

The `MarkupHelper#markup` method does everything we need here.

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

 .diff-file
   .diff-content
-    - if gitlab_markdown?(@blob.name)
+    - if markup?(@blob.name)
       .file-content.wiki
-        = preserve do
-          = markdown(@content)
-    - elsif markup?(@blob.name)
-      .file-content.wiki
-        = raw markup(@blob.name, @content)
+        = markup(@blob.name, @content)
     - else
       .file-content.code.js-syntax-highlight
         - unless @diff_lines.empty?