Remove FORBIDDEN_KEYS and check_forbidden_key from entropy-tss

[ameba23]

Im pretty sure these are a relic from subgroups and no longer used.

I think this function:

entropy-core/crates/threshold-signature-server/src/validator/api.rs

Line 378 in a1e3698

pub fn check_forbidden_key(key: &str) -> Result<(), ValidatorErr> {

is only called in tests, and is the only thing that uses the forbidden kvdb keys.