refactor: remove legacy mcpb-manifest.schema.json

Removed the non-versioned mcpb-manifest.schema.json from schemas/
directory. All schemas are now explicitly versioned.

Available schemas:
- mcpb-manifest-v0.1.schema.json
- mcpb-manifest-v0.2.schema.json
- mcpb-manifest-v0.3.schema.json
- mcpb-manifest-latest.schema.json

Consumers should use -latest or specific version to be explicit
about which schema version they're validating against.

Author: joan-anthropic