ima: Remove ima_policy file before directory

The removal of ima_dir currently fails since ima_policy still exists, so
remove the ima_policy file before removing the directory.

Fixes: 4af4662f ("integrity: IMA policy")
Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Cc: <stable@vger.kernel.org>
Acked-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>