refactor(config): remove unused import of secp256k1::SecretKey

This commit cleans up the `config.rs` file by removing the unused import of `secp256k1::SecretKey`, enhancing code clarity and maintainability.

Author: michaelgpt

app/src/actors/chain/config.rs

@@ -249,5 +249,3 @@ pub enum ConfigError {
     #[error("Invalid performance target: {0}")]
     InvalidPerformanceTarget(String),
 }
-
-use secp256k1::SecretKey;