malloc: Use _int_free_chunk in tcache_thread_shutdown

[Wilco Dijkstra]

Hi Florian,

> Does this mean we can eliminate tcache_shutting_down?

We already can since we removed tcache_init from __libc_free.
So if we only call free() after tcache_thread_shutdown() there is
no need to do anything else.

Cheers,
Wilco