schemas: remove unnecessary ml-kem oneOf indirection

[cpu]

oneOf with a single option doesn't offer us much, and comes at the cost of ugly code generation (e.g. interface{} types in generated Go code). Instead, just use a direct $ref of the one group definition in use.

Using go-jsonschema's output as an example:

Before:

  type MlkemTestSchemaJson struct {
  	// ...
  	TestGroups []interface{} `json:"testGroups"`
  }

After:

  type MlkemTestSchemaJson struct {
  	// ...
  	TestGroups []MLKEMTestGroup `json:"testGroups"`
  }