navbar: Remove unnecessary overflow attributes from #tab_bar CSS.

Prior to commit eb4a2b9d4e the center
area of the navbar was based on a structure that appended crumbs or
"tabs" as <li>s, forming a tab_bar and a tab_list.

However, in eb4a2b9d4e we apply a new
style and structure to the navbar which lets go of the convention of
tabs. Hence, we'd like to purge the tab_bar and tab_list labels from
our code base. This commit pushes us towards that goal by simplifying
the future merge of the two CSS labels.

Currently the #tab_bar is just a container for the #tab_list and the
 #tab_list handles the overflow attributes and so these do not serve
any purpose.

static/styles/zulip.scss

@@ -1459,8 +1459,6 @@ div.focused_table {
     width: 100%;
     z-index: 2;
     padding-top: 0px;
-    overflow: hidden;
-    text-overflow: ellipsis;
     float: left;
     letter-spacing: normal;
     #tab_list {