Remove the obsolete logic in img_from_target_files.py.

img_from_target_files.py used to handle the case that a given TF.zip not
containing the image entries under IMAGES/. That is only the case for
pre-Lollipop releases.

Also unzip the needed files only since we know that for sure now.

Test: img_from_target_files.py with an existing bullhead-TF.zip gives
      the same bullhead-img.zip.
Change-Id: I892379ba388df80ae63be9d3ce647fbb77fd4753