MDL-80313 behat: remove dupe type attribute

Because of Masterminds/html5-php#242

that duplicate attribute is causing problems. As far as we don't
need it, we proceed to remove leaving only the type=submit that
will make NonJS behat tests (BrowserKit) happy.

course/templates/activity_list.mustache

@@ -31,7 +31,7 @@
                    placeholder="{{#str}}search, core {{/str}}" {{#query}} value="{{query}}" autofocus {{/query}}
             >
             <div class="input-group-append">
-                <button type="submit" class="btn btn-outline-secondary rounded-right" type="button">
+                <button type="submit" class="btn btn-outline-secondary rounded-right">
                     <i class="icon fa fa-search fa-fw m-0" aria-hidden="true"></i>
                     <span class="sr-only">{{#str}}submitsearch, course {{/str}}</span>
                 </button>