releasetools: Remove more images out of secondary payload.

This CL additionally removes boot/dtbo/modem/vbmeta_* images out of
secondary payload. We essentially only keep system_other.img and
bootloader images there.

For Pixel devices, this additionally saves ~80MiB (mostly because of
the removal of boot and radio images).

Bug: 140771390
Test: ota_from_target_files \
          --include_secondary --skip_postinstall \
          -i input-target_files-1.zip \
          input-target_files-2.zip \
          output-ota.zip
Test: python -m unittest test_ota_from_target_files
Change-Id: If47b27c52b3547a4cc86223a988c53960bc6af40