Remove unused arenas_extend() function declaration.
The arenas_extend() function was renamed to arenas_init() in commit
8bb3198f72fc7587dc93527f9f19fb5be52fa553, but its function declaration
was not removed from jemalloc_internal.h.in.
diff --git a/include/jemalloc/internal/jemalloc_internal.h.in b/include/jemalloc/internal/jemalloc_internal.h.in
index d3b94c0..c1cccd6 100644
--- a/include/jemalloc/internal/jemalloc_internal.h.in
+++ b/include/jemalloc/internal/jemalloc_internal.h.in
@@ -465,7 +465,6 @@
void *bootstrap_malloc(size_t size);
void *bootstrap_calloc(size_t num, size_t size);
void bootstrap_free(void *ptr);
-arena_t *arenas_extend(unsigned ind);
unsigned narenas_total_get(void);
arena_t *arena_init(unsigned ind);
arena_tdata_t *arena_tdata_get_hard(tsd_t *tsd, unsigned ind);