Revert "config: BoardConfigReservedSize: Prevent overlayfs on remount"

Reserving 8MB does not guarantee that free blocks will be >= 1%, so not
only this doesn't work properly, but also breaks builds for some
devices.

This reverts commit 3940fb42.

Test: breakfast kebab && WITH_GMS=true m
Change-Id: I330570dc6fc9efca5629358a0b149f131d36ba18