Redundant/Useless TrieDB instances

[hsyodyssey]

• Since no functions in trie\database.go are used as the current middle layer for MT to submit data to Disk, all instances of TrieDB are theoretically useless.
• Also, since the structure of MT nodes is different from that of the original MPT, the functions of reference counting in trie\database.go are invalid.

But there are still a lot of functions calls to trie\database.go in the codebase.

For example, in the core/blockchain.go , NewBlockChain function creates an instance of trie\database.go\Database. And this instance is called multiple times in the core/blockchain.go , such as:

triedb := bc.stateCache.TrieDB()

Even some function about reference counting of nodes is called, we can find it in this line: https://github.com/scroll-tech/go-ethereum/blob/zkrollup-smt/core/blockchain.go#L1243.

Fortunately, since MT has not submitted any data to this instance, its dirties array is always empty. Therefore, the relevant call is always a direct return, which is why the current code works without problems.

If we want to support both MPT and MT, these areas may need some attention.