Remove legacy registry Go types and converters

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 — stacklok/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: stacklok/toolhive-catalog#1215, stacklok/toolhive-core#105, stacklok/toolhive#5273
• Tracking on consumer side: stacklok/toolhive#5279

First check stacklok/toolhive#5279 and the catalog reference to confirm both consumers no longer import registry/converters. Then inspect registry/types/ and registry/converters/, including the converter tests, and remove them once reverse dependencies are gone. Run go build ./... and go test ./...; done means both commands pass and the listed directories and imports are absent.