Remove href attribute from tab link to prevent duplicate event on enter.

Because Discourse doesn't preventDefault on links with an href of "#"
this tab-able link was firing a click event when enter was pressed. This
had the unfortunate side effect of breaking the history, making routing
and navigation unreliable.
Merge pull request #2898 from kellec/remove_focus_link_href

Remove href attribute from tab link to prevent duplicate event on enter.