[IPV6] MIP6: Kill unnecessary ifdefs.

Kill unnecessary CONFIG_IPV6_MIP6.

o It is redundant for RAW socket to keep MH out with the config then
  it can handle any protocol.
o Clean-up at AH.

Signed-off-by: Masahide NAKAMURA <nakam@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>