Revert "Fix possible lockup in an atexit handler"

* src/estream.c (_gpgrt_fflush): Do not use the in_atexit flag.

We need to research how we can better handle atexit handlers and
stream locking; in particilar for stdout and stderr. For now we keep
the code but disable it.

• Fixes-commit: 50f0c56bdd36f7f709a62503942ac55399dafb9c