ALSA: hda: Drop useless WARN_ON()

Since the commit 97cc2ed2 ("ALSA: hda - Fix yet another i915
pointer leftover in error path") cleared hdac_acomp pointer, the
WARN_ON() non-NULL check in snd_hdac_i915_register_notifier() may give
a false-positive warning, as the function gets called no matter
whether the component is registered or not.  For fixing it, let's get
rid of the spurious WARN_ON().

Fixes: 97cc2ed2

 ("ALSA: hda - Fix yet another i915 pointer leftover in error path")
Cc: <stable@vger.kernel.org>
Reported-by: Kouta Okamoto <kouta.okamoto@toshiba.co.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>