Remove junk filling in tcache_bin_flush_small().

Junk filling is done in arena_dalloc_bin_locked(), so arena_alloc_junk_small()
is redundant. Also, we should use arena_dalloc_junk_small() instead of
arena_alloc_junk_small().
Merge pull request #108 from wqfish/dev

Remove junk filling in tcache_bin_flush_small().