Leave these checks up to the callers, the only uses of this function
(indirect, through meta_wayland_seat_get_grab_info) are
[wl_shell|xdg]_surface.move/resize/show_window_menu.

In move/resize it makes sense to check for a button being pressed, because
we must expect a button release event. However for xdg_surface.show_window_menu
we 1) don't strictly need further events and 2) we must account for press+release
event pairs being processed at once in the compositor before the client sees
the former.

That is eg. the case of touchpad 2nd/3rd button tap emulation, multifinger
taps will emit the event pair at once, so when the client manages to request
xdg_surface.show_window_menu, it'll be too late in the compositor side, so the
request is ignored.

https://bugzilla.gnome.org/show_bug.cgi?id=764519