platform_utils: delete unused FileDescriptorStreams APIs

Now that we've converted the few users of this over to subprocess APIs,
we don't need this anymore.  It's been a bit hairy to maintain across
different operating systems, so there's no desire to bring it back.

Using multiprocessing Pool to batch things has been working better in
general anyways.

Change-Id: I10769e96f60ecf27a80d8cc2aa0d1b199085252e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297682


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