SELinux: remove redundant exports

Remove the following exported SELinux interfaces:
selinux_get_inode_sid(inode, sid)
selinux_get_ipc_sid(ipcp, sid)
selinux_get_task_sid(tsk, sid)
selinux_sid_to_string(sid, ctx, len)

They can be substitued with the following generic equivalents
respectively:
new LSM hook, inode_getsecid(inode, secid)
new LSM hook, ipc_getsecid*(ipcp, secid)
LSM hook, task_getsecid(tsk, secid)
LSM hook, sid_to_secctx(sid, ctx, len)

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>
Acked-by: James Morris <jmorris@namei.org>
Reviewed-by: Paul Moore <paul.moore@hp.com>