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.

include/jemalloc/internal/ctl.h

@@ -35,7 +35,6 @@ struct ctl_arena_stats_s {
 	bool			initialized;
 	unsigned		nthreads;
 	const char		*dss;
-	ssize_t			lg_dirty_mult;
 	ssize_t			decay_time;
 	size_t			pactive;
 	size_t			pdirty;