net: Remove unused net-checksum.c file

The common checksum functions were moved to net/checksum.c in commit
7200ac3c7c8eefe574193b49eeff09f120e11ec7 but the original net-checksum.c
was never deleted from the source tree.  Remove it now since all users
of the checksum functions link against net/checksum.o and net-checksum.c
is not even compiled anymore.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
