releasetools: Remove the obsolete path in GetImage().

We must have created the images (system.img, system.map etc) prior to
calling ota_from_target_files.py (added by commit
2e0d8fcf, into Lollipop).

Remove the obsolete suppport for handling "old" target_files zips that
don't have such images. This simplies the logic for BuildSystem() and
BuildVendor(), which now would only be called by
add_img_to_target_files.py itself.

Test: Generating full and incremental OTAs give the same results, w/ and
      w/o this CL.
Change-Id: I0ea243d09d3378249d7982701ae4ec339b04b7b8