Remove BITMAP_USE_TREE.

Remove tree-structured bitmap support, in order to reduce complexity and
ease maintenance.  No bitmaps larger than 512 bits have been necessary
since before 4.0.0, and there is no current plan that would increase
maximum bitmap size.  Although tree-structured bitmaps were used on
32-bit platforms prior to this change, the overall benefits were
questionable (higher metadata overhead, higher bitmap modification cost,
marginally lower search cost).

include/jemalloc/internal/bitmap_structs.h

@@ -10,19 +10,8 @@ struct bitmap_info_s {
 	/* Logical number of bits in bitmap (stored at bottom level). */
 	size_t nbits;
 
-#ifdef BITMAP_USE_TREE
-	/* Number of levels necessary for nbits. */
-	unsigned nlevels;
-
-	/*
-	 * Only the first (nlevels+1) elements are used, and levels are ordered
-	 * bottom to top (e.g. the bottom level is stored in levels[0]).
-	 */
-	bitmap_level_t levels[BITMAP_MAX_LEVELS+1];
-#else /* BITMAP_USE_TREE */
 	/* Number of groups necessary for nbits. */
 	size_t ngroups;
-#endif /* BITMAP_USE_TREE */
 };
 
 #endif /* JEMALLOC_INTERNAL_BITMAP_STRUCTS_H */