doc: remove note about Nova plugin framework
doc: remove note about Nova plugin framework

The architecture doc alludes to a Nova plugin framework which is now
considered obsolete.

See oslo-incubator review I5552b5625e26f4821464807743545ec6f5117f2b
(commit 4b78868) for more background.

Change-Id: Id170510bc3147af94941c993ffe6997c418d4be4

luogangyi/Ceilometer-oVirt,tanglei528/ceilometer,cernops/ceilometer,maestro-hybrid-cloud/ceilometer,fabian4/ceilometer,NeCTAR-RC/ceilometer,openstack/ceilometer,ityaptin/ceilometer,redhat-openstack/ceilometer,m1093782566/openstack_org_ceilometer,redhat-openstack/ceilometer,pkilambi/ceilometer,openstack/aodh,eayunstack/ceilometer,pczerkas/aodh,tanglei528/ceilometer,sileht/aodh,Juniper/ceilometer,idegtiarov/ceilometer,openstack/aodh,MisterPup/Ceilometer-Juno-Extension,m1093782566/openstack_org_ceilometer,Juniper/ceilometer,r-mibu/ceilometer,mathslinux/ceilometer,isyippee/ceilometer,MisterPup/Ceilometer-Juno-Extension,froyobin/ceilometer,sileht/aodh,luogangyi/Ceilometer-oVirt,idegtiarov/ceilometer,openstack/ceilometer,pczerkas/aodh,chungg/aodh,NeCTAR-RC/ceilometer,froyobin/ceilometer,r-mibu/ceilometer,NeCTAR-RC/ceilometer,fabian4/ceilometer,tanglei528/ceilometer,isyippee/ceilometer,maestro-hybrid-cloud/ceilometer,ityaptin/ceilometer,eayunstack/ceilometer,cernops/ceilometer,mathslinux/ceilometer,pkilambi/ceilometer,chungg/aodh