datapath: Remove check for shared skbs.

We never allow shared skbs to be present inside of the OVS datapath
but the presence of a check in the core makes this less clear.  Since
the check is very old and no longer relevant, drop it.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>