common: drop the unused prune() of {safe,mutable}_item_history.

common: drop the unused prune() of {safe,mutable}_item_history.

The presence of these never-used methods obscure bug investigation.
For instance, `safe_item_history` is used to record all messenger's
addresses. As we never lock reading from it in `_conn_prefix()`, we
expect they live as long their corresponding messenger instances.
When such an item becomes corrupted, presence of `prune()` imposes
verification the method is never called – this effort could have
been avoided if we hadn't have the dangerous method.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>