Revert "unpack_bootimg: Export 'vendor_ramdisk' for vendor_boot v4" am: f85a2fd564 am: ccf44dde20

Original change: https://android-review.googlesource.com/c/platform/system/tools/mkbootimg/+/2181573

Change-Id: I3941095a0a29062eff508bf8274afd1ebc530a0e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>