Remove snapshot migrator class (#3088)

cmd/downloader/generator/commands/generate_body_snapshot.go

...downloader/generator/commands/generate_header_snapshot.go

cmd/downloader/generator/commands/verify_headers.go

cmd/state/commands/verify_headers_snapshot.go

cmd/state/verify/verify_headers_snapshot.go

eth/backend.go

turbo/snapshotsync/bodies_snapshot.go

turbo/snapshotsync/headers_snapshot.go

turbo/snapshotsync/snapshot_builder.go

turbo/snapshotsync/wrapdb.go