chore: do not override theme.openapi with ConfigReferenceDocs node type

[tatomyr]

What/Why/How?

Getting rid of overwriting theme.openapi with the ConfigReferenceDocs node type it's going to come from the Redocly config package soon.

Reference

Testing

Screenshots (optional)

Check yourself

• [ ] Code changed? - Tested with redoc/reference-docs/workflows (internal)
• [ ] All new/updated code is covered with tests
• [ ] New package installed? - Tested in different environments (browser/node)

Security

• [ ] Security impact of change has been considered
• [ ] Code follows company security practices and guidelines