Remove security_group_handler

Now that nova's security groups are more plugable and decoupled from the
database the security_group_handler code is not very useful so this patch
removes it.

A little info about the security_group_handler:
The security_group_handle code was added to provide a hook into nova security
groups so that one could get security group add/delete/update notification
and proxy them somewhere else (i.e quantum). Trying to actually using this
method opens one up to several transactional issues because in the current
implemenation the security group is commited to the nova database before
security_group_handler is called.

Implements blueprint: remove-security-group-handler

Change-Id: I45047fe8dbb81555505e03309838910113080c37
Merge "Remove security_group_handler"