Drop unused change_i2c_mux()

Change-Id: I3ac39441746d739ac19e831bb67c76405c24ba27
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/5925
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Idwer Vollering <vidwer@gmail.com>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>