core-backtrace-python: omit build_id for C callbacks

Seems like the build_id would always be generated from
"_ctypes/callbacks.c" file, which is doomed to fail (or in worse
case, to capture wrong file accidentally named like this at the
current path) when processed by hash_text_file, e.g.:

> Generating core_backtrace
> Unable to open '_ctypes/callbacks.c': No such file or directory.
> hash_text_file failed for '_ctypes/callbacks.c'
> Core backtrace is generated and saved, 436 bytes

extract from /var/log/messages encountered in connection to bug
https://bugzilla.redhat.com/show_bug.cgi?id=873569.

But I am not omniscient and there can be counterexamples to my
assumption.

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>
Signed-off-by: Martin Milata <mmilata@redhat.com>