Remove unused files

[Mirko-von-Leipzig]

0xMiden/miden-base#1301 added some minor changes to the account tree. This apparently breaks some test utilities here:

In miden-node, this breaks test code in crates/block-producer/src/test_utils/store.rs but the fix should be relatively easy

^^ Already handled by #807

---

I've also noticed we still have some pre-mempool vestiges floating around in the block-producer i.e. state-view and some other src files that are in-tree but not part of the module system. We should remove those.

Similarly, we should check if this test_utils/store.rs is actively used - I suspect not.

[Mirko-von-Leipzig]

We could also try https://github.com/cpg314/cargo-workspace-unused-pub to shake out public items which are unused within the workspace.