Remove the unused plugins framework

This framework was merged a year ago and AFAICT hasn't seen use beyond
the two initial wikimedia extensions for Nova.

The framework basically allows a way for a single plugin to register API
extensions and notification hooks. Both of these can be done by directly
using config opts like osapi_compute_extension and notification_driver
so this framework really only helps if we expected to (and wanted to)
lots of these plugins in the wild.

Nova is the only project using this and we don't have an active
maintainer listed in the MAINTAINERS file, so it seems like it's time to
remove it from the incubator.

Change-Id: I5552b5625e26f4821464807743545ec6f5117f2b
Merge "Remove the unused plugins framework"