index-pack: drop type_cas mutex

The type_cas lock lost all of its callers in f08cbf60fe (index-pack:
make quantum of work smaller, 2020-09-08), so we can safely delete it.
The compiler didn't alert us that the variable became unused, because we
still call pthread_mutex_init() and pthread_mutex_destroy() on it.

It's worth considering also whether that commit was in error to remove
the use of the lock. Why don't we need it now, if we did before, as
described in ab791dd138 (index-pack: fix race condition with duplicate
bases, 2014-08-29)? I think the answer is that we now look at and assign
the child_obj->real_type field in the main thread while holding the
work_lock(). So we don't have to worry about racing with the worker
threads.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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