fix locking in efi_secret_unlink()

We used to need securityfs_remove() to undo simple_pin_fs() done when
the file had been created and to drop the second extra reference
taken at the same time.  Now that neither is needed (or done by
securityfs_remove()), we can simply call simple_unlink() and be done
with that - the broken games with locking had been there only for the
sake of securityfs_remove().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>