Replace it by a new degeneralize_tba(), that use the same tricks as
degeneralize().

* src/tgba/tgbatba.cc, src/tgba/tgbatba.hh: Delete.
* src/tgba/Makefile.am: Adjust.
* src/tgbaalgos/degen.cc, src/tgbaalgos/degen.hh: Implement
a degeneralize_tba() function sharing its code
with degeneralize().
* src/tgbatest/ltl2tgba.cc: Rename -D to -DT so that we can pass it the
same option as -DS.
* src/tgbatest/degenid.test, src/tgbatest/emptchk.test,
src/tgbatest/emptchke.test, src/tgbatest/ltlcounter.test,
src/tgbatest/ltlcross.test, src/tgbatest/spotlbtt.test,
src/tgbatest/ltl2tgba.test: Adjust.
* src/tgbatest/det.test, src/tgbatest/emptchk.test: Adjust numbers to
the smaller output.
* src/saba/sabacomplementtgba.cc, src/saba/sabacomplementtgba.hh,
src/tgbaalgos/minimize.cc, src/tgbaalgos/neverclaim.cc,
src/tgbaalgos/neverclaim.hh, src/tgbaalgos/postproc.cc,
src/tgbatest/randtgba.cc, src/tgbatest/complementation.cc,
wrap/python/spot.i, wrap/python/tests/ltl2tgba.py,
src/sabatest/sabacomplementtgba.cc: Adjust to the removal
of tgba_tba_proxy, using degeneralize_tba() if needed.