Remove PUBLISHER_BASE_URL from default set of environment variables

[davidjoy]

Today the PUBLISHER_BASE_URL environment variable is included in our core set of variables defined in frontend-platform:

https://github.com/edx/frontend-platform/blob/master/src/config.js#L40

Publisher itself - https://github.com/edx/frontend-app-publisher - is not part of any Open edX release.

As far as I can tell, the PUBLISHER_BASE_URL is only used in the Support Tools MFE:

https://github.com/edx/frontend-app-support-tools/blob/c316ef2d62f481356aec6308815c9969bb471993/src/supportHeader/Header.jsx#L26

That MFE is also not a part of any Open edX release.

My recommendation would be that we have the support tools MFE manually add PUBLISHER_BASE_URL to its config using mergeConfig and then remove it from MFEs in Open edX releases and from frontend-platform - MFEs not in Open edX releases yet can keep it. Auditing environment variables should be part of our MFE inclusion process.