Cannot remove fields

[clintonb]

A recent attempt to remove a field from a config model failed: https://github.com/edx/edx-platform/pull/14939. Please document the process for removing fields.

[cpennington]

This is a subclass of the general problem: we don't have a good way to remove fields from our database models, because that requires a backwards-incompatible migration.

[cpennington]

(And backwards-incompatible migrations are also rolling-release incompatible migrations)

[doctoryes]

https://openedx.atlassian.net/browse/TE-2059