Retire pruned cache

[antiochp]

Our "prune list" implementation maintains an internal "prune cache".

• The prune list consists of roots of fully pruned subtrees.
• The cache contains all pos beneath these roots.
• We check if a pos was pruned by looking in the cache.

We can improve this without needing to maintain an internal cache.

To check if a pos is pruned we can look for the "next" pruned subtree and check if pos is beneath it.
We only need to consider the "next" subtree due to the append-only PMMR semantics.
If the next subtree does not include pos then we know that no other subtree will either.

[antiochp]

This is a small optimization inlining only the logic required to get the height.
We don't need to calculate peaks and then throw them away.