In 3fe74f1c, fixed_size_pool was changed in order to
help memcheck to detect "potential" memory leaks. In a
multithreaded context, this could raise false alarm. To
solve this, we proprose 2 policies for the pool, one with
the check and one without.

* spot/misc/fixpool.cc: deleted ...
* spot/ltsmin/ltsmin.cc, spot/ltsmin/spins_kripke.hh,
spot/mc/deadlock.hh, spot/misc/Makefile.am,
spot/misc/fixpool.cc, spot/misc/fixpool.hh,
spot/priv/allocator.hh, spot/ta/tgtaproduct.cc,
spot/ta/tgtaproduct.hh, spot/twa/twaproduct.cc,
spot/twa/twaproduct.hh, tests/core/mempool.cc: Here.