eventpoll: drop dead bool return from ep_remove_epi()

ep_remove_epi() always returns true -- the "can be disposed"
answer was meaningful back when the dying-check lived inside the
pre-split __ep_remove(), but after that check moved to ep_remove()
the return value is just noise. Both callers gate on it
unconditionally:

  if (ep_remove_epi(ep, epi))
      WARN_ON_ONCE(ep_refcount_dec_and_test(ep));

  dispose = ep_remove_epi(ep, epi);
  ...
  if (dispose && ep_refcount_dec_and_test(ep))
      ep_free(ep);

Make ep_remove_epi() return void, drop the dispose local in
eventpoll_release_file(), and the useless conditionals at both
callers. No functional change.

Link: https://patch.msgid.link/20260423-work-epoll-uaf-v1-9-2470f9eec0f5@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>