API: libcrmcluster: Drop crm_join_nack_quiet enum value

Neither this value nor the check for feature set 3.17.0 were ever
needed. By simply increasing the feature set minor version in 866a03fb,
we avoided the possibility of sending a join nack to an older node. The
older client will reject the join offer due to incompatible feature set
and exit fatally. The join offer message comes before the join nack
message, so we never reach the nack.

This breaks API backward compatibility.

Ref T793

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>