Remove canonical for 404 and 403 pages

Problem/Motivation

The default config for 404 and 403 pages specifies [site:url] for the canonical url.

I wonder if this is correct. I look at http://google.com/page-dont-exist and they don't set an rel links in the header.

Steps to reproduce

Install the module with default config.

Proposed resolution

There should be a way to disable some (or all) metatag headers in a configuration. This is not the same as disabling the configuration as this will just fall back to the global config

Remaining tasks

- [ ] Discuss
- [ ] Implement

User interface changes

- [ ] Add way to specify removal of tags -e.g. by typing "" or a checkbox next to each setting to remove or an extra element where the admin can specify which tags to remove

API changes

Data model changes