Remove ratio-based purging.

Make decay-based purging the default (and only) mode.

Remove associated mallctls:
- opt.purge
- opt.lg_dirty_mult
- arena.<i>.lg_dirty_mult
- arenas.lg_dirty_mult
- stats.arenas.<i>.lg_dirty_mult

This resolves #385.

src/jemalloc.c

@@ -1053,25 +1053,6 @@ malloc_conf_init(void)
 			}
 			CONF_HANDLE_UNSIGNED(opt_narenas, "narenas", 1,
 			    UINT_MAX, false)
-			if (strncmp("purge", k, klen) == 0) {
-				int i;
-				bool match = false;
-				for (i = 0; i < purge_mode_limit; i++) {
-					if (strncmp(purge_mode_names[i], v,
-					    vlen) == 0) {
-						opt_purge = (purge_mode_t)i;
-						match = true;
-						break;
-					}
-				}
-				if (!match) {
-					malloc_conf_error("Invalid conf value",
-					    k, klen, v, vlen);
-				}
-				continue;
-			}
-			CONF_HANDLE_SSIZE_T(opt_lg_dirty_mult, "lg_dirty_mult",
-			    -1, (sizeof(size_t) << 3) - 1)
 			CONF_HANDLE_SSIZE_T(opt_decay_time, "decay_time", -1,
 			    NSTIME_SEC_MAX);
 			CONF_HANDLE_BOOL(opt_stats_print, "stats_print", true)