Revert "Ensure to kill all processes and reap them before unmounting"

This reverts commit ff490878.

Bug: 425867970
Test: do Mainline update, observe there's no ramdump during the next
reboot
Flag: EXEMPT revert
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:d5cf77e7501fb63e2e965b289ae8522d5b8d0609)
Merged-In: I9c289cc71b85ed3184384f73fb57876dbbbde0b5
Change-Id: I9c289cc71b85ed3184384f73fb57876dbbbde0b5