nsheap was an horror full of virtual functions required to
customize gtec to implement inclusion-based emptiness-check
in GreatSPN support.  Since this support has been removed, we
can remove the nsheap cruft as well.  Note that nsheap was
also used in emptinessta for no good reason (the code from
emptinessta was simply copied from gtec without cleanup).

* src/tgbaalgos/gtec/nsheap.cc, src/tgbaalgos/gtec/nsheap.hh:
Delete.
* src/tgbaalgos/gtec/Makefile.am: Adjust.
* src/taalgos/emptinessta.cc, src/taalgos/emptinessta.hh,
src/taalgos/tgba2ta.cc, src/tgbaalgos/gtec/ce.cc,
src/tgbaalgos/gtec/gtec.cc, src/tgbaalgos/gtec/gtec.hh,
src/tgbaalgos/gtec/status.cc, src/tgbaalgos/gtec/status.hh:
Use a simple unordered_map.