Remove middleware ratelimits from v3 api

The ratelimit middleware isn't really useful. That is pointed out
by https://review.openstack.org/#/c/34821/. And it didn't get any
object from mail-list:
http://lists.openstack.org/pipermail/openstack-dev/2013-November/020291.html

This patch remove ratelimit middleware and limits extension. The config option
'api_rate_limit' is only valid in v2 api. The pipeline factory for v3 api won't
check that option anymore. For compatibility, if user is still using old
'api-paste.init', pipeline factory will ignore to load ratelimit middleware, and
print warning message to user for notice ratelimit is deprecated in v3 api.

Closes-bug: 1255471
DocImpact

Change-Id: Ifeae0504e11089f95e4d8af58bcb7372dca87f81
Merge "Remove middleware ratelimits from v3 api"