markdown: Stop setting target="_blank".

This setting is being overridden by the frontend since the last
commit, and the security model is clearer and more robust if we don't
make it appear as though the markdown processor is handling this
issue.

Co-authored-by: Tim Abbott <tabbott@zulipchat.com>
Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

static/js/markdown.js

@@ -444,13 +444,13 @@ exports.initialize = function (realm_filters, helper_config) {
           + '\n</pre></div>\n\n\n';
     };
 
-    // Our links have title= and target=_blank
+    // Our links have title=
     r.link = function (href, title, text) {
         title = title || href;
         if (!text.trim()) {
             text = href;
         }
-        const out = '<a href="' + href + '"' + ' target="_blank" title="' +
+        const out = '<a href="' + href + '"' + ' title="' +
                   title + '"' + '>' + text + '</a>';
         return out;
     };