Remove keyid_hash_algorithms

[znewman01]

go-tuf added keyid_hash_algorithms as a field in 2fbbd60 (June 2020); see python-tuf#848 for context. However, it never made it into the spec, and now it’s not needed due to TAP-12 (which is accepted). python-tuf removed it (python-tuf#1014); we should too.

[znewman01]

@trishankatdatadog brings up a good point: be careful about backwards compat here! Old clients may expect this field.

So we should:

• stop expecting it (client side) immediately
• after a minor version bump, stop producing it (repo side) for new repos
• have an option to keep producing it indefinitely, if a repo ever had it (they probably don't want to break old clients)

[rdimitrov]

Closing since the code base changed and this is already fixed.

Thanks for raising this 👍