And remove redundant code that sets _enable variables
This is a partial backport of secp256k1#1079
https://github.com/bitcoin-core/secp256k1/pull/1079/changes/0d4226c051415d12384586ae938499975a248cab
https://github.com/bitcoin-core/secp256k1/pull/1079/changes/fabd579dfa85711e96e4012d20352b8ee8f2a9d1