delta_performer: continue to validate operation hashes

Checking the install operation hashes is skipped if the metadata
signature is not provided to support some transition in ChromeOS. This
does not apply to CoreOS and metadata signature is going away for now
but it doesn't hurt to continue to check other hashes.
delta_generator: remove support for writing metadata signatures
cleanup: remove remaining references to metadata signatures.

The functional code related to metadata signatures is gone now, this
expunges the lingering support code. Mostly it is just a lot of glue
passing the value from one data structure to the next.