Explicitly forbid the use of cancellations

[ahankinson]

See: https://github.com/rism-digital/muscat-incipit-corrections/issues/247#issuecomment-2324910820

I indeed tend to read "It is not necessary..." as allowing the cataloger to nonetheless explicitly cancel the previous key signature if s/he so chooses, so IMO that sort of coding should either be accepted as an equally correct alternative, or this option should explicitly be excluded by a MUST NOT.