Do not dehugify when purging

Giving the advice MADV_DONTNEED to a range of virtual memory backed by
a transparent huge page already causes that range of virtual memory to
become backed by regular pages.

include/jemalloc/internal/hpa_hooks.h

@@ -10,7 +10,6 @@ struct hpa_hooks_s {
 	void (*unmap)(void *ptr, size_t size);
 	void (*purge)(void *ptr, size_t size);
 	bool (*hugify)(void *ptr, size_t size, bool sync);
-	void (*dehugify)(void *ptr, size_t size);
 	void (*curtime)(nstime_t *r_time, bool first_reading);
 	uint64_t (*ms_since)(nstime_t *r_time);
 	bool (*vectorized_purge)(void *vec, size_t vlen, size_t nbytes);