Remove encryption elements that are not used

[dericed]

Except for the ones used for DRM already in use in WebM.

[dericed]

Moving @robUx4's issue from ietf-wg-cellar/ebml-specification#85 to the right repo.

[robUx4]

That would be

• signature ones: ContentEncoding\ContentEncryption\ContentSignature, ContentEncoding\ContentEncryption\ContentSigKeyID, ContentEncoding\ContentEncryption\ContentSigAlgo, ContentEncoding\ContentEncryption\ContentSigHashAlgo.
• compression ones, ContentEncoding\ContentEncoding\ContentCompression, ContentEncoding\ContentCompression\ContentCompAlgo, ContentEncoding\ContentCompression\ContentCompSettings

The compression ones are definitely used in Matroska, even more formats are proposed (#423).

The signatures are unlikely to be used. We can either marked them as unusable (maxver=0) or deprecated (maxver=4).