[dhcp] Revert various patches

[dhcp] Revert various patches

A recent patch series breaks compatibility with various common DHCP
implementations.

Revert "[dhcp] Don't consider invalid offers to be duplicates"
This reverts commit 905ea567534bbb67b83b2be41bfcd42209681e6a.

Revert "[dhcp] Honor PXEBS_SKIP option in discovery control"
This reverts commit 620b98ee4b9d8fa87c56bfa72b4391c1eed9dce6.

Revert "[dhcp] Keep multiple DHCP offers received, and use them intelligently"
This reverts commit 5efc2fcb602864e82de2cd7414e3828f589034e5.

Signed-off-by: Michael Brown <mcb30@ipxe.org>