Remove abort handler from the backtrace generator

Remove abort handler from the backtrace generator

The abort handler masks the exit status of the backtrace generator by
capturing the abort signal from the backtrace handler and exiting with
zero. Because the output of the backtrace generator is meant to be piped
to `bt_test.py`, its exit status is unimportant and is currently
ignored.

The abort handler calls `exit(3)` which is not asynchronous-signal-safe
and calling it in this context is undefined behavior [0].

Closes ticket 21026.

[0] https://www.securecoding.cert.org/confluence/x/34At