desktop-shell: Clear out missing listeners

The keyboard focus listener, caps changed and pointer focus listener
were missing when destroying the seat. These are necessary to avoid
using weston_desktop object even if it was destroyed, which happens due
to a focus out event and ultimately handled by the keyboard focus notify
callback.

Once the seats are destroyed (and implictly the focus handlers) we're
safe to destroy weston_desktop object as well.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>