In fetch_net_wm_check_window(), before updating the wmspec_check_window, a
check is performed to verify a 15s difference between last_wmspec_check_time
and the current monotonic time.

The comment suggests that this check is done to ensure that it doesn't check
for a new check window repeatedly over and over again. While that was the case
origionally, currently the last_wmspec_check_time only gets updated when
wmspec_check_window is set, which is already checked earlier, making the time
check useless.

This check causes issues on cold boots where gtk4 applications are not able
to obtain the wmspec_check_window until 15 seconds after boot, making gtk
unable to check for extended wm_hints during that time.

Fixes: #6558