Preserve taxonomy_forums field when uninstalling forum module

Is there an equivalent of field_delete_instance in d8? that used to have the 'cleanup' flag.
So we could call that, if something else used it - it would stick around, else it would be removed.

The forum uninstall tests can be removed, as they are to check the field is gone.
The uninstall test I'm not sure on, its making sure that taxonomy can be removed once forum has been - from memory.

Updated issue summary.

6: drupal8.forum-module.2032699-6.patch queued for re-testing.

Removed field deletion and it's part of tests.

So, I believe this issue runs counter to the expectations we've set up for config dependencies. What should happen is:

1. You cannot remove a field until all data in it has been purged. This should already be the case following #2198429: Make deleted fields work with config synch.
2. For correct dependency management, and to unblock #2140511: Configuration file name collisions silently ignored for default configuration, all of the forum module's supplied configuration should be removed when the module is uninstalled. This means its vocabulary, its node type, and its field. (See #2224581-7: Delete forum data on uninstall.)
3. The taxonomy field should have a dependency on the taxonomy vocabulary, ensuring that the field data must be purged first, then the field deleted, then the vocabulary deleted. If the field data has not been purged, then the module cannot be uninstalled.
4. If other config entities have added the taxonomy field, they too are subject to the same rules. (Field data must be purged before the field is deleted.)

So, I think this issue is actually a wonfix, and what we should do instead is clean up all our data for #2224581: Delete forum data on uninstall.

Totally agree to won't fix in light of #2224581: Delete forum data on uninstall