Do not show tab open warning during session closing

Fixes: GNOME/epiphany#1093
3 jobs for fix-multiple-window-session in 8 minutes and 44 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  .Pre
passed #591095
check-code-style

00:02:17

passed #591096
flatpak-scanbuild

00:04:16

 
  Build
passed #591097
flatpak

00:04:26