fs_mgr: Drop the context mount option from non-selinux builds

Transitioning devices may have the context option present in the
fstab, even if we're not doing selinux builds.
Since we don't set or even create the contexts on those builds,
this will make mount fail, and either the mountpoint ends up absent,
or boot fails entirely

Change-Id: Ic5cc7f5632cc3d62459dfaaf7719d662737e1641