PointerConstraint: remove overly tight assert

This assert is incorrect if Xwayland is enabled and an Override Redirect
window steals the keyboard focus from the parent surface.

It also seems likely to be hit if a Wayland client attempts to use a
pointer constraint on a subsurface. I don't think a pointer constraint
on a subsurface is likely to work entirely correctly and I don't know of
any Wayland clients that try such a thing. We can't let them crash river
by trying though.

(cherry picked from commit 5d1fc034bc)

river/PointerConstraint.zig

@@ -105,8 +105,6 @@ pub fn maybeActivate(constraint: *PointerConstraint) void {
 pub fn updateState(constraint: *PointerConstraint) void {
     const seat: *Seat = @ptrFromInt(constraint.wlr_constraint.seat.data);
 
-    assert(seat.wlr_seat.keyboard_state.focused_surface == constraint.wlr_constraint.surface);
-
     constraint.maybeActivate();
 
     if (constraint.state != .active) return;