ofproto: Break out monitor deletion code

Break out monitor deletion code into a new function
flow_monitor_delete which is paramatised over the id of
the monitor to delete.

This is in preparation for supporting OpenFlow1.4 flow monitor requests
with delete and modify commands.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>