Remove root from MerkleTree

[ernestognw]

Fixes #4946

After discussing, we considered it rather unnecessary to store the root of the tree along with it, and its core functionality is enough for anyone to build use cases where the root is tracked.

With this change the user may decide to keep track of the root for purposes that may not require on-chain availability (e.g. a whitelist that stores roots every N insertions) or that may require a lookup mechanism (e.g. Bitmaps) for which storing the root is already an opinionated decision.

We can consider adding a default recommendation, such as #4911.

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[Amxx]

That works, but thecking the event is a bit counterintuitive IMO, and it doesn't "show" the need for devs to store the root "outside the library"

I modified the mock so that the old test don't need to me modify at all. I think minimizing the diff is preferable.