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
Merge "doc: remove note about Nova plugin framework"