remove Category from the type system

Category was a manually-assigned label with no remaining functional
purpose (decay was the only behavior it drove, and that's gone).
Remove the enum, its methods, category_counts, the --category search
filter, and all category display. The field remains in the capnp
schema for backwards compatibility but is no longer read or written.

Status and health reports now show NodeType breakdown (semantic,
episodic, daily, weekly, monthly) instead of categories.

Co-Authored-By: ProofOfConcept <poc@bcachefs.org>

src/store/ops.rs

@@ -251,14 +251,6 @@ impl Store {
         Ok((hubs_capped, to_delete.len()))
     }
 
-    pub fn category_counts(&self) -> HashMap<&str, usize> {
-        let mut counts = HashMap::new();
-        for node in self.nodes.values() {
-            *counts.entry(node.category.label()).or_insert(0) += 1;
-        }
-        counts
-    }
-
     /// Update graph-derived fields on all nodes
     pub fn update_graph_metrics(&mut self) {
         let g = self.build_graph();