Revert "Port _FileDescriptorStreamsNonBlocking to use poll()"

This reverts commit 1e01a744.

Not all platforms support select.poll() currently it seems.
At least macOS's Python 2 doesn't (while macOS Python 3 does).

Lets back this out for the existing release series and once we
start repo-2 which is Python 3-only, we can put this back in.

Change-Id: I205206b0fa4fe2d755f4fbc6ec683ad125f27cc2
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/253072


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>