Remove logic to fake /system in overlayfs

It is better to guarantee that a /system or / entry will be present in
first stage mount than it is to maintain the code to fake an entry if
its not present in the input fstab.

Test: adb-remount-test.sh on blueline
Change-Id: I8aa3e704903b8abf06b1c63be071913a9de58eb3