Remove duplicate metadata type declarations from `src/models.ts`

[joshtrichards]

While looking at some version discrepancies like this one:

end_to_end_encryption/src/models.ts

Line 43 in 9bfa09f

version: '2.0'

... I noticed that src/models.ts contains metadata types that duplicate the declarations in src/models/metadata.d.ts. These have drifted -- for example, Metadata.version only permits '2.0', while the implementation supports and emits '2.1'.

Migrate remaining consumers, primarily test fixtures, to the more authoritative IRawMetadata, IRawRootMetadata, and related types, then remove the duplicate declarations from src/models.ts.