ofproto: Additional simplifications.

ofproto: Additional simplifications.

This commit finishes the removal of asynchronous flow table operations
begun in the previous commit, by removing ofoperation and ofopgroup
entirely and all of the code that depended on them.  Following this commit,
all the internal documentation and comments should again be consistent and
correct.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>