Kconfig: Drop CONFIG_CMD_DS4510_MEM

This option is only used in one driver and is not enabled by any board. It
does not seem worth having the ability to remove this part of the support.

Drop the option.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Heiko Schocher <hs@denx.de>