fix: #1243: Remove "other" generic metadata

[brogdonm]

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As denoted in #1243 , the metadata extensions of actions was never fully specified in the documents and therefore can't be truly supported. This PR removes it completely.

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment.

[ok-nick]

For some additional background, the c2pa docs specify * $$assertion-metadata-map-extension, where * usually signifies any field. In this case, it represents a CDDL group socket where if no socket is specified it means "the choice is empty."

Names that start with a single "$" are "type sockets", starting out
as an empty type, and intended to be extended via "/=". Names that
start with a double "$$" are "group sockets", starting out as an
empty group choice, and intended to be extended via "//=". In either
case, it is not an error if there is no definition for a socket at
all; this then means there is no way to satisfy the rule (i.e., the
choice is empty).

[gpeacock]

I misread the spec on this, thanks for the catch. This is technically a breaking change for the API, but I doubt it will effect anything other than our unit tests.