datapath: Remove tunnel header caching.

Tunnel caching was added to reduce CPU utilization on TX path
by caching packet header, So performance gain is directly proportional
to number of skbs transferred.  But with help of offloads skb are getting
larger. So there are less number of skbs.  Therefore header caching does
not shows similar gains we seen in past.  And now kernel 3.6 has removed
dst caching from networking which makes header caching even more tricky.
So this commit removes header caching from OVS tunnelling.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>