Remove platform_app shell_data_file:lnk_file read access.

Not sure what denial originally motivated adding this
access, but drop it and see if it resurfaces.  platform_app
is still permissive_or_unconfined() so this should not break
anything.

Change-Id: Ia4418080e3477346fa48d23b4bb5d53396ed5593
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>