removed redundant FixedHash trait, fixes #4029 (#4866)

ethstore/src/dir/memory.rs

@@ -67,7 +67,7 @@ impl KeyDirectory for MemoryDirectory {
 	fn unique_repr(&self) -> Result<u64, Error> {
 		let mut val = 0u64;
 		let accounts = self.accounts.read();
-		for acc in accounts.keys() { val = val ^ ::util::FixedHash::low_u64(acc) }
+		for acc in accounts.keys() { val = val ^ acc.low_u64() }
 		Ok(val)
 	}
 }