Remove `ToDNSSlice`, `FromProtoWithDefault`, and
`FromProtoSliceWithDefault` now that all their callers are gone. All
protobufs but one have migrated from DnsNames to Identifiers.
Remove TODOs for the exception, `ValidationRecord`, where an identifier
type isn't appropriate and it really only needs a string.
Rename `corepb.ValidationRecord.DnsName` to `Hostname` for clarity, to
match the corresponding PB's field name.
Improve various comments and docs re: IP address identifiers.
Depends on #8221 (which removes the last callers)
Fixes#8023
7214b285e4