Replace spot::ltl_file by a rewritten spot::ltl::ltl_file.

* src/tgba/tgbafromfile.cc, src/tgba/tgbafromfile.hh: Delete these
files.
* src/tgba/Makefile.am: Remove them.
* src/ltl/ltlparse/ltlfile.hh, src/ltl/ltlparse/ltlfile.cc: New
files.
* src/ltl/ltlparse/Makefile.am: Add them.
* bench/scc-stats/stats.cc, bench/split-product/cutscc.cc: Rewrite
using the new class.

src/tgba/Makefile.am

@@ -44,7 +44,6 @@ tgba_HEADERS = \
   tgbabddcoredata.hh \
   tgbabddfactory.hh \
   tgbaexplicit.hh \
-  tgbafromfile.hh \
   tgbakvcomplement.hh \
   tgbascc.hh \
   tgbaproduct.hh \
@@ -69,7 +68,6 @@ libtgba_la_SOURCES = \
   tgbabddconcreteproduct.cc \
   tgbabddcoredata.cc \
   tgbaexplicit.cc \
-  tgbafromfile.cc \
   tgbakvcomplement.cc \
   tgbaproduct.cc \
   tgbareduc.cc \