Remove age_counter from hpa_central

Before this commit we had two age counters: one global in HPA central
and one local in each HPA shard. We used HPA shard counter, when we are
reused empty pageslab and HPA central counter anywhere else. They
suppose to be comparable, because we use them for allocation placement
decisions, but in reality they are not, there is no ordering guarantees
between them.

At the moment, there is no way for pageslab to migrate between HPA
shards, so we don't actually need HPA central age counter.

include/jemalloc/internal/hpa.h

@@ -31,8 +31,6 @@ struct hpa_central_s {
 	size_t eden_len;
 	/* Source for metadata. */
 	base_t *base;
-	/* Number of grow operations done on this hpa_central_t. */
-	uint64_t age_counter;
 
 	/* The HPA hooks. */
 	hpa_hooks_t hooks;