dhcp: remove lookup_opcode()

The lookup_opcode() function is incorrect. It was mishandling
DHCP_OPTION_PAD, which does not have a following length byte.

Also, the while condition is reading 'op' value which never
changes. This may result in reads beyond packet size.

Since this function is unused the best fix is to remove it.

Reveiwed-by: Vlad Zolotarov <vladz@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>