[ada] remove dead dwarf frame eh support

[Olivier Hainque]

Richard Henderson <rth@redhat.com> writes:
> I realize that a replacement is in progress, but in the meantime
> this code references header files that don't exist.  Ok to kill?

Yes. It was there as a first attempt to use dwarf2 unwinding for exception
propagation with gcc 2.8.1 a long time ago but we are now using the higher
level _Unwind API for this purpose.

Olivier