net: remove CONFIG_MCAST_TFTP

No mainline board enables CONFIG_MCAST_TFTP and there have been
compilation issues with the code for some time. Additionally, it has a
potential buffer underrun issue (reported as a side note in
CVE-2018-18439).

Remove the multicast TFTP code but keep the driver API for the future
addition of IPv6.

Cc: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Signed-off-by: Chris Packham <judge.packham@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>