Remove all functionality from openproject-plugins

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Add deprecation warning when required by Bundler

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Note deprecation in README

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Merge pull request #7 from opf/feature/integrate-op-plugins-plugin

Gently kill this plugin