[parity crypto]: remove unused depend rustc_hex (#337)

Author: niklasad1

parity-crypto/Cargo.toml

@@ -30,7 +30,6 @@ pbkdf2 = "0.3.0"
 subtle = "2.2.1"
 zeroize = { version = "1.0.0", default-features = false }
 rand = "0.7.2"
-rustc-hex = "2.0"
 
 [dev-dependencies]
 criterion = "0.3.0"