This warning

--8<---------------cut here---------------start------------->8---
warning: onCleanup: error caught while executing cleanup function:
'python_ipc_popen2_reset' undefined near line 78, column 30
  "'python_ipc_popen2_reset' undefined"
--8<---------------cut here---------------end--------------->8---

is caused by "pkg unload symbolic".

We fix it by closing file descriptors and waitpid directly in the
cleanup function instead of using private/python_ipc_popen2_reset.

Fixes #1160.

* inst/private/python_ipc_popen2.m: Close file descriptors and waitpid
directly in the cleanup function.
* inst/private/python_ipc_popen2_reset.m: Remove file.

Just in case the onCleanup triggers early and they have been closed
or something.