Removes JsonSchema reference resolution when parsing a doc.

[irvinesunday]

This PR:

• Partially closes Post v2 Preview Checklist #1613
• Removes JsonSchema reference resolution step when parsing a doc.
• Splits JsonSchema reference resolution from the reference host document setting into separate classes.
• Updates some tests in line with the above changes.
• Deletes some tests that were not relevant in light of the above changes.

NB: By removing JsonSchema reference resolution, we now don't validate whether the JsonSchema references concrete instances are defined in the components/schemas section of a document (locally or externally).