A few things leftover from schema

TAG_INVALID

The TAG_INVALID description says:

During a manifest upload, if the tag in the manifest does not match the uri tag, this error will be returned.

Manifests no longer contains tags, so we should update this. A couple options:

1. Remove the error entirely.
2. Use this error if the client tries to use a tag that doesn't match the expected format. Ref Tags are essentially undefined in the spec #52

Signatures

There are 8 uses of the word signature in the spec. AFAIK, manifest signatures aren't really a thing after docker v2 schema 1.

For example, the MANIFEST_UNVERIFIED description says:

During manifest upload, if the manifest fails signature verification, this error will be returned.

I assume we'd want to just remove that.

The rest of the instances of "signatures" refer to JWS payloads and such that don't make sense anymore, so we want to drop those descriptions and make sure the surrounding paragraphs still make sense.