Metadata: remove duplicate API

The Metadata class was originally designed to provide convenience
accessors for all child ImageMetadata. However, this significantly
bloats the API and makes maintenance fragile and frustrating.

Especially with the addition of MetadataWrappers, there was no need to
repeat each ImageMetadata API addition 3 times.

Thus, the duplicated API has been removed and all invocations changed
simply to get(index).<method>.