ALSA: lx6464es: Remove unused function in pci/lx6464es/lx_core.c

Remove unused function in pci/lx6464es/lx_core.c.

This eliminates the following warning in pci/lx6464es/lx_core.c:
sound/pci/lx6464es/lx_core.c:144:5: warning: no previous prototype for ‘lx_plx_mbox_read’ [-Wmissing-prototypes]
sound/pci/lx6464es/lx_core.c:172:5: warning: no previous prototype for ‘lx_plx_mbox_write’ [-Wmissing-prototypes]
sound/pci/lx6464es/lx_core.c:494:5: warning: no previous prototype for ‘lx_dsp_es_check_pipeline’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>