Allow rebuild sepolicy even though the image does not exist.

The `--rebuild-sepolicy` option should work when the target
files folder `ODM/` or `VENDOR/` exists. We should not
assume that there is always an image to remove.

Bug: 270529723
Test: m otatools; Create merged builds with rebuild_sepolicy.
Change-Id: I93aeea2cd084fdc7f8179732d77a78bb1d144616