Nuke ‘MyFamily’

For reasons discussed in legacy/trac#5565 (moved), the ‘MyFamily’ option should go away:

• It is probably harmful for clients to refuse to use two relays operated by the same honest entity, and malicious relay operators will not use MyFamily honestly.
• Honest relay operators have a hard time setting MyFamily correctly on all of their relays.
• Relay family information bloats up microdescriptors, likely for no benefit.
• Groups of relays operated by the same entity can be recognized tolerably well (e.g. for metrics purposes) using their ContactInfo string instead (modulo the possible need for fuzzy matching of some sort).

This change requires (assuming I didn't forget anything):

• a proposal, spec change, and consensus method to remove relay family information from microdescriptors;
• a corresponding code change for Tor dirauths;
• a code change to clients to make them ignore relay family information (if they receive it, e.g. by turning off UseMicrodescriptors) by default, and warn if the user tries to re-enable use of relay family information without disabling UseMicrodescriptors.