don't need implement RequestContextSerializer and JsonPayloadSerializer in celometer

[Liusheng]

For oslo_messaging has implement the RequestContextSerializer and JsonPayloadSerializer [1], we don't need these two class definition in ceilometer [2] any more.

[1] http://git.openstack.org/cgit/openstack/oslo.messaging/tree/oslo_messaging/serializer.py#n66

[2] http://git.openstack.org/cgit/openstack/ceilometer/tree/ceilometer/messaging.py#n25

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/194148
Committed: https://git.openstack.org/cgit/openstack/ceilometer/commit/?id=3cdeb25bd01aae5e82ada8bbf82dd2dc23a4346e
Submitter: Jenkins
Branch: master

commit 3cdeb25bd01aae5e82ada8bbf82dd2dc23a4346e
Author: LiuSheng <email address hidden>
Date: Mon Jun 22 21:51:55 2015 +0800

    Clean the re-implemented serializers in Ceilometer

    The RequestContextSerializer and JsonPayloadSerializer have been
    implemented in oslo_messaging, This change remove the re-implemented
    serializers in ceilometer/messaging.py

    Change-Id: I871ff34a8e1a140273fd10fe1fbb539ff864c9f7
    Closes-Bug: 1467524