Remove fake tab from tabbing order

This tab has no reason to receive focus, since it exists for cosmetic
reasons only.

app/views/devise/shared/_tab_single.html.haml

 = gl_tabs_nav({ class: 'new-session-tabs gl-border-0' }) do
-  = gl_tab_link_to tab_title, '#', { item_active: true, class: 'gl-cursor-default!', tab_class: 'gl-bg-transparent!' }
+  = gl_tab_link_to tab_title, '#', { item_active: true, class: 'gl-cursor-default!', tab_class: 'gl-bg-transparent!', tabindex: '-1' }