Removed built-in OpenAPI support

[sliverc]

Description of the Change

Planning a new major release 8.0.0 I thought it is time to also remove built-in OpenAPI support as it has been deprecated for quite a while now and it also does not work with the newest django-filter version anymore.

In case there are any concerns removing it please let me know.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[sliverc]

@n2ygk Just saw that I missed removing some code, pushed that now. In case you consider this for merging, could you also test in an actual project that schema generation still works with drf-spectacular-json-api and these changes?
I do not have an actual project, so won't be able to check on that.

[n2ygk]

@n2ygk Just saw that I missed removing some code, pushed that now. In case you consider this for merging, could you also test in an actual project that schema generation still works with drf-spectacular-json-api and these changes? I do not have an actual project, so won't be able to check on that.

Looks like breaking changes to setuptools that I have to resolve first... It has to do with setuptools no longer accepting local filesystem paths.

Tested locally without using tox and it looks good.

[n2ygk]

Looks good. I tested my demo project to confirm that nothing breaks.

Not sure if we want to document the new way using drf-spectacular-jsonapi but that is certainly appropriate for a separate documentation PR if deemed useful.