releasetools: Stop copying images from RADIO/ to IMAGES/.

We've added support in brillo_update_payload that allows additionally
looking for images under RADIO/ in the given target_files zips [1]. This
avoids having duplicate radio images in target_files zips.

Also adjust the unittest in test_ota_from_target_files.py to cover this
path.

As a result of this CL, the radio images will no longer appear in the
image archive (i.e. <target>-img.zip) as well - they are less useful
anyway because we have packed only the _updatable_ pieces that are part
of full bootloader/radio images.

Bug: 77218220
Test: `python -m unittest test_ota_from_target_files`
Test: `python -m unittest test_add_img_to_target_files`
Test: `m dist` produces the same full OTA package
Test: Build marlin-userdebug in internal branch. Check the image zip.
Change-Id: I05579480f0bb9ab90aaeecf75969ee29b6904ad6