Tidy up extent quantization.

Remove obsolete unit test scaffolding for extent quantization.  Remove
redundant assertions.  Add an assertion to
extents_first_best_fit_locked() that should help prevent aligned
allocation regressions.

include/jemalloc/internal/extent_externs.h

@@ -11,10 +11,6 @@ extent_hooks_t	*extent_hooks_get(arena_t *arena);
 extent_hooks_t	*extent_hooks_set(arena_t *arena, extent_hooks_t *extent_hooks);
 
 #ifdef JEMALLOC_JET
-typedef size_t (extent_size_quantize_t)(size_t);
-extern extent_size_quantize_t *extent_size_quantize_floor;
-extern extent_size_quantize_t *extent_size_quantize_ceil;
-#else
 size_t	extent_size_quantize_floor(size_t size);
 size_t	extent_size_quantize_ceil(size_t size);
 #endif