Remove legacy registry Go types and converters

[rdimitrov]

Summary

PR #105 deleted the legacy schema and validator but intentionally kept the legacy Go types (Registry, ImageMetadata, RemoteServerMetadata) and the registry/converters/ package because external consumers still depended on them. Once those consumers are off, this repo should drop them too.

Scope

• registry/types/ — Registry, ImageMetadata, RemoteServerMetadata
• registry/converters/ — the converter package and its tests

Blocked on

• stacklok/toolhive — Align registry API with toolhive-registry-server v0.1 surface toolhive#5279 (aligns the registry API with toolhive-registry-server's v0.1 surface and removes the registry/converters import from toolhive's API handlers)
• stacklok/toolhive-catalog — cmd/catalog/update_metadata.go uses registry/converters for provenance verification

This issue can land once both consumers stop importing the package.

Acceptance criteria

• registry/types/ and registry/converters/ removed.
• No reverse dependencies in stacklok/toolhive or stacklok/toolhive-catalog.
• go build ./... and go test ./... green.

References

• Landed: chore: remove legacy registry format toolhive-catalog#1215, chore: remove legacy registry schema and validator #105, Drop legacy registry schema from release artifacts toolhive#5273
• Tracking on consumer side: Align registry API with toolhive-registry-server v0.1 surface toolhive#5279