[ALSA] hda-codec - remove duplicate controls in alc268 test mixer

I've just noticed that there are a handful of duplicate controls in the
ALC268 test model mixer.  This patch (against alsa-driver 1.0.16) removes
them.

Signed-off-by: Jonathan Woithe <jwoithe@physics.adelaide.edu.au>
Signed-off-by: Takashi Iwai <tiwai@suse.de>