net: Remove CONFIG_BOOTP_DHCP_REQUEST_DELAY

This option is not in use anywhere and the documentation implies it's
for some very old and unlikely to be seen currently issues.  Rather than
update the code so the CONFIG symbol could be easily in Kconfig, remove
the code.

Cc: Ramon Fried <rfried.dev@gmail.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Ramon Fried <rfried.dev@gmail.com>