Strip the netrc hack; apparently it only works on some versions of
Python, and the bots have many different versions.

Instead, we will actually emit the modified netrc as a new file without
comments or newlines, then read that directly from the vanilla netrc
module.

BUG=chromium:664664
TEST=None

Change-Id: I245759e2eae4e6ddcdffa0aaef59866a62bd0240
Reviewed-on: https://chromium-review.googlesource.com/411416
Commit-Queue: Daniel Jacques <dnj@chromium.org>
Reviewed-by: Ryan Tseng <hinoka@chromium.org>
Reviewed-by: Michael Moss <mmoss@chromium.org>