lib/ofpbuf: Remove 'l7' pointer.

lib/ofpbuf: Remove 'l7' pointer.

Now that we don't need to parse TCP flags from the packet after
extraction, we usually do not need the 'l7' pointer any more.  When
needed, ofpbuf_get_tcp|udp|sctp|icmp_payload() or ofpbuf_get_l4_size()
can be used instead.

Removal of 'l7' was requested by Pravin for the DPDK datapath work, as
it simplifies packet parsing a bit.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>