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As reboot_notify callback is no longer used by the codec core, let's
get rid of the unused code.  Conexant codec needs a slight code change
as it used to call the reboot_notify at the codec removal, too.

BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=214045
Link: https://lore.kernel.org/r/20210813081230.4268-4-tiwai@suse.de

Signed-off-by: Takashi Iwai <tiwai@suse.de>