datapath: Remove unnecessary flow variable.

Patch fixes following warning:
datapath/linux/flow_table.c:580:40: warning: symbol 'flow' shadows an earlier one
datapath/linux/flow_table.c:558:24: originally declared here

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