chore(levm): remove unused bigint dep from levm (#4060)

**Motivation**

With the change from bigint to malachite for modexp, bigint is now an
unused dependency.

fixes #4057

Author: edg-l

Cargo.lock

@@ -27,7 +27,6 @@ p256 = { version = "0.13.2", features = [
 ] }
 sha2 = "0.10.8"
 ripemd = "0.1.3"
-num-bigint = "0.4.6"
 malachite = "0.6.1"
 lambdaworks-math = "0.11.0"
 bls12_381 = { git = "https://github.com/lambdaclass/bls12_381", branch = "expose-fp-struct", features = [