private/python_ipc_popen2: Fixes warning after unloading package.

[alexvong243f]

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.