The "mrproper" target is still looking for build time generated keys in
the kernel root directory instead of certs directory. Fix the path and
remove the names of the files which are no longer generated.

Fixes: cfc411e7 ("Move certificate handling to its own directory")
Signed-off-by: NNayna Jain <nayna@linux.ibm.com>
Reviewed-by: NStefan Berger <stefanb@linux.ibm.com>
Reviewed-by: NJarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: NMimi Zohar <zohar@linux.ibm.com>