Remove experimental_hpa_max_purge_nhp option

It is unused and no longer needed: hpa_dirty_mult, hpa_purge_threshold,
hpa_min_purge_interval_ms, and hpa_min_purge_delay_ms now provide finer
control over the purge rate.

test/unit/hpa_sec_integration.c

@@ -36,8 +36,6 @@ static hpa_shard_opts_t test_hpa_shard_opts = {
     false,
     /* min_purge_interval_ms */
     5,
-    /* experimental_max_purge_nhp */
-    -1,
     /* purge_threshold */
     PAGE,
     /* min_purge_delay_ms */