Remove duplicate deprecated spec.JsonSchemaValidator (#666)

There were two JsonSchemaValidator types in the codebase:
io.modelcontextprotocol.spec.JsonSchemaValidator (deprecated) and
io.modelcontextprotocol.json.schema.JsonSchemaValidator (the one in
use). As noted by a maintainer in the issue, the deprecated one in the
spec package should be removed.

It was already annotated @deprecated pointing to the json.schema
variant, and nothing references it: no imports, no fully-qualified
usages, and no wildcard imports of the spec package resolve to it. All
production and test code already uses
io.modelcontextprotocol.json.schema.JsonSchemaValidator.

Fixes #666

Author: k-krawczyk