Draft: Remove validate_core_schema

[sydney-runkle]

This improves perf significantly in pydantic, so we no longer have a use for this in pydantic-core.

cc @davidhewitt, before I continue:

• Can we remove the self schema gen
• Can I comb through tests using this function and either remove them or modify the schema errors?

[sydney-runkle]

Decided we don't need this in pydantic-core until we've decided skipping this step is stable.