dpif-provider: Get rid of redundant operations.

The dpif provider 'operate' call duplicates all of the features available
from the 'flow_put', 'flow_del', and 'execute' calls, yielding redundant
code in providers that support both mechanisms.  This change drops the
latter calls in favor of making every dpif provider support 'operate'.
The result is code that is overall less duplicative.

It might make sense to do the same with flow_get but so far 'operate'
doesn't support flow_get.

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