fix(miner): clippy error (#4793)

Description
---
Removes unused function in miner

Motivation and Context
---
Clippy

How Has This Been Tested?
---
No clippy error

applications/tari_miner/src/difficulty.rs

@@ -23,7 +23,7 @@
 use std::convert::TryInto;
 
 use tari_app_grpc::tari_rpc::BlockHeader as grpc_header;
-use tari_core::{blocks::BlockHeader, large_ints::U256, proof_of_work::sha3_difficulty};
+use tari_core::{blocks::BlockHeader, proof_of_work::sha3_difficulty};
 use tari_utilities::epoch_time::EpochTime;
 
 use crate::errors::MinerError;
@@ -81,13 +81,6 @@ impl BlockHeaderSha3 {
     }
 }
 
-/// This will provide the difficulty of the hash assuming the hash is big_endian
-fn big_endian_difficulty(hash: &[u8]) -> Difficulty {
-    let scalar = U256::from_big_endian(hash); // Big endian so the hash has leading zeroes
-    let result = U256::MAX / scalar;
-    result.low_u64()
-}
-
 #[cfg(test)]
 pub mod test {
     use chrono::{DateTime, NaiveDate, Utc};