fs_mgr: overlayfs stop adding rootcontext= to mount

By adding the root context, we cause an issue with kernel security
profile, the kernel can kick back overlay mounts because the sepolicy
is not initialized.

observable error when we have root context setup:

SELinux: Unable to set superblock options before the security\
         server is initialized
init: [libfs_mgr]__mount(source=overlay,target=/system,\
         type=overlay,upperdir=/cache/overlay/system/upper)=-1:\
	 Invalid argument

root context was added initially due to paranoia, so the side effects
are non existent unless upperdir sepolicy is adjusted.

Test: manual
Bug: 109821005
Change-Id: Id218add0a32df3b2a4626d8cb8aa783760c78cc4