releasetools: Drop the support for creating image archive from dir.

Taking an extracted target_files dir input is only needed for
add_img_to_target_files.py, in particular for the call from build
system. For the case of img_from_target_files.py, it goes much faster
with zip2zip (3m vs 7s in my local test). This CL drops the unneeded
code path in preparation for the switch to zip2zip.

Test: `python build/make/tools/releasetools/img_from_target_files.py \
         -v aosp_taimen-target_files.zip aosp_taimen-img-test.zip`
Change-Id: I7f85d7d629d6fd1621a082f94a11cac7c3e6bbf2