Drop special handling of app_data_file in mls constraints.

This was a legacy of trying to support per-app level isolation
in a compatible manner by blocking direct open but permitting
read/write via passing of open files over Binder or local sockets.
It is no longer relevant and just confusing to anyone trying to use
the mls support for anything else.

Change-Id: I6d92a7cc20bd7d2fecd2c9357e470a30f10967a3
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>