xwm: Init window link after removing it

In future code the window link will end up in a state where it may or may
not be on the unpaired_window_list and we'll want to go from that state
to one where it's definitely not on the list.

Initting the list after removal (in these two places) allows us to
unconditionally remove it later.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>