Revert "Try to have /system mounted as read-only on secondary ROMs"

This reverts commit 59171739e5d12768422c5b8b50bb1b929977df05.

Conflicts:
	multirom.c

This causes SuperSU to remount everything read-only on secondary ROMs.
Thank god SuperSU is open-source, so that I can find out why is it
doing this, come up with a fix and make it work properly.

Oh. Wait. It's not.

┛(ò.ó)┛彡┻━┻