Revert "update ota_from_target_files to handle mounting/unmounting for backupscript"

This reverts commit cc343dd39fa7589802bb530aff171ffb0b79b422.
Revert "Modular backuptool.sh.  Executes backup and restore methods defined in arbitrary /system/addon.d/*.sh scripts."

This reverts commit 64641b7ee492c10b64d37b96cabdd8a174793883.