media: saa7164: Remove unused functions

saa7164_cmd_signal() last use was removed in 2009 by
commit 39e469ab ("V4L/DVB (12940): SAA7164: IRQ / message timeout
related change")

saa7164_buffer_display() was added in 2010 by
commit add3f580 ("[media] saa7164: convert buffering structs to be more
generic")
but is unused.

Remove them.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>