fix: Remove LeveldbStore from stdlib_merkle_tree

[phated]

Description

This removes the leveldb_store from stdlib_merkle_tree and removes the workaround in join_split_example that linked leveldb when it wasn't actually used.

leveldb is packaged poorly (ref google/leveldb#606 and google/leveldb#920) and mostly unmaintained.

For these reasons, I don't think that leveldb should be part of barretenberg's public interface.

Checklist:

• I have reviewed my diff in github, line by line.
• Every change is related to the PR description.
• I have linked this pull request to the issue(s) that it resolves.
• There are no unexpected formatting changes, superfluous debug logs, or commented-out code.
• There are no circuit changes, OR specifications in /markdown/specs have been updated.
• There are no circuit changes, OR a cryptographer has been assigned for review.
• I've updated any terraform that needs updating (e.g. environment variables) for deployment.
• The branch has been rebased against the head of its merge target.
• I'm happy for the PR to be merged at the reviewer's next convenience.
• New functions, classes, etc. have been documented according to the doxygen comment format. Classes and structs must have @brief describing the intended functionality.
• If existing code has been modified, such documentation has been added or updated.

[phated]

This has been changed to remove leveldb from BB completely.