Remove the unused plugins framework

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

The framework basically allows a way for a single plugin to register API
extensions and notification drivers. 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.

Change-Id: I09a11f9931ee0436a56e8b0d925683b54f73b104
Merge "Remove the unused plugins framework"