ime: fix segfault when IME is killed

ime: fix segfault when IME is killed

Fixes up e530f43.

When IME (e.g. fcitx5) is killed, relay->input_method is destroyed and
then relay->input_method->keyboard_grab is destroyed, which causes null
pointer dereference and crashes labwc.

Possible solutions are:
- Let wlroots keep emitting keyboard grab as `data` from keyboard grab's
  destroy handler just like before
- Let wlroots destroy keyboard grab before input method
- Let compositor store keyboard grab as relay->keyboard_grab

But let's just revert the change in e530f43 for now.