Remove unneeded 'slave' flag for /data_mirror/data_ce/null

Remove the 'slave' mount flag that was added by commit ef927522
(https://r.android.com/2095463) because it doesn't actually do anything
in this context.  MS_SLAVE can only be used to change the propagation
type of an existing mount, and the kernel ignores it if MS_BIND is also
specified, due to the way the various high-level operations that the
mount() system call can do are prioritized.

The reason that the /data/user/0 mount gets propagated into /data_mirror
anyway is because the /data mount has the "shared" propagation type.  In
the above-mentioned commit I had assumed the default Linux mount
semantics, but actually Android applies the "shared" propagation type to
everything (see SetupMountNamespaces() in init/mount_namespace.cpp).

Test: Booted Cuttlefish and verified (via /proc/self/mountinfo) that
      /data/data is still bind-mounted to both /data/user/0 and
      /data_mirror/data_ce/null/0.
Bug: 156305599
BYPASS_INCLUSIVE_LANGUAGE_REASON=commit message mentioning removed code
Change-Id: Idc45d8dcb3a21d4e8e2e72f4d4dda7286f898127