[dhcp] Remove obsolete dhcp_chaddr() function

As of commit 03f0c23 ("[ipoib] Expose Ethernet-compatible eIPoIB
link-layer addresses and headers"), all link layers have used
addresses which fit within the DHCP chaddr field.  The dhcp_chaddr()
function was therefore made obsolete by this commit, but was
accidentally left present (though unused) in the source code.

Remove the dhcp_chaddr() function and the only remaining use of it,
unnecessarily introduced in commit 08bcc0f ("[dhcp] Check for matching
chaddr in received DHCP packets").

Reported-by: Wissam Shoukair <wissams@mellanox.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>