[IPV4]: Remove three declarations of unimplemented methods and correct a typo in include/net/ip.h

These three declarations in include/net/ip.h are not implemented
anywhere:

ip_mc_dropsocket(), ip_mc_dropdevice() and ip_net_unreachable().

Also, correct a comment to be "Functions provided by ip_fragment.c"
(instead of by ip_fragment.o) in consistency with the other comments
in this header.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>