Fix: Backup stopped due the missing /boot (from Pilu1978)

The originallí commit is overwrited somewhere

Pilu1978's commit: https://github.com/pilu1978/TeamWin_Recovery_X8/commit/e571d315608dbee2b46bc7528526bc76377f41c5
The merged commit: https://github.com/OldXperiaTeam/android_bootable_recovery-twrp/commit/79bd8e17da4f4335e442e0574e899af41ab3b554

Remove some code to solve the Backup/Restore error (Backup stopped due
the missing /boot partition)