remove the clone operation of code_cache (#8334)

* Some tiny modifications.
1. fix some typo in the comment.
2. sort the order of methods in 'impl state::Backend for StateDB`

* Remove the clone of code_cache, as it has been done in clone_basic.

ethcore/src/state/account.rs

@@ -424,7 +424,6 @@ impl Account {
 	pub fn clone_dirty(&self) -> Account {
 		let mut account = self.clone_basic();
 		account.storage_changes = self.storage_changes.clone();
-		account.code_cache = self.code_cache.clone();
 		account
 	}