present/wnmd: Remove dead check from present_wnmd_check_flip

present_wnmd_toplvl_pixmap_window returns a window with the same window
pixmap, so the check could never fail.

Reviewed-by: Roman Gilg <subdiff@gmail.com>
4 jobs for xwl_present_check_flip2-window-pixmap-check in 4 minutes and 12 seconds (queued for 3 seconds)