cmd: remove duplicate DHCPv6 Kconfig definitions

cmd: remove duplicate DHCPv6 Kconfig definitions

Remove duplicate definition of

* DHCP6_PXE_CLIENTARCH
* DHCP6_PXE_DHCP_OPTION
* DHCP6_ENTERPRISE_ID

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Fixes: da24eb553279 ("Merge patch series "BOOTP/DHCPv4 enhancements"")
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Fixes: 5eb1b7843811 ("Merge patch series "test/py: enable HTTP testing"")
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>