releasetools: Fix a mismatching close.

There's a mismatch in WriteABOTAPackageWithBrilloScript().

  temp_zip_file = tempfile.NamedTemporaryFile()
  ...
  common.ZipClose(temp_zip_file)

It's benign since common.ZipClose() happens to be calling
"temp_zip_file.close()". This CL moves the use of tempfile to
common.MakeTempFile(), so that the tempfile will be cleaned up
automatically as part of the call to common.Cleanup(). (Not fixing the
close() directly, since the nearby lines will be refactored into another
function shortly.)

Also remove one assert in the same function, which trivially holds in
the current code.

Test: Generate an A/B OTA.
Change-Id: I53b375d1150820de718dec0ead55abf5f4951071