Exceptions: Do not set verbose terminate handler

Exceptions: Do not set verbose terminate handler

All of this is entirely redundant nowadays. libstdc++ defaults to
setting the verbose terminate handler since (almost) forever. So all we
end up doing is setting something that is already the default.

Remove it.

Rationale:

 - libstdc++ (gcc) switched to __gnu_cxx::__verbose_terminate_handler a
   long time ago [1].

 - libc++ (clang) has its own verbose terminate handler. And above code
   snippet never worked there [2]:

Example:

```

struct my_exception : public std::runtime_error {
  my_exception() : std::runtime_error("Nope, not with me") {}
};

int main() { throw my_exception(); }
```

libstdc++:
terminate called after throwing an instance of 'my_exception'
  what():  Nope, not with me
zsh: abort (core dumped)  ./a.out

libc++:
terminating with uncaught exception of type my_exception: Nope, not with me

[1] https://gcc.gnu.org/onlinedocs/libstdc++/manual/termination.html

[2] The DEAL_II_HAVE_VERBOSE_TERMINATE check fails for libc++ because
    (of course) no __gnu_cxx:: namespace is available...