keys: cleanup build time module signing keys

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: cfc411e7fff3 ("Move certificate handling to its own directory")
Signed-off-by: Nayna Jain <[e-mail]>
Reviewed-by: Stefan Berger <[e-mail]>
Reviewed-by: Jarkko Sakkinen <[e-mail]>
Signed-off-by: Mimi Zohar <[e-mail]>

Makefile

···
 		  debian snap tar-install \
 		  .config .config.old .version \
 		  Module.symvers \
-		  signing_key.pem signing_key.priv signing_key.x509	\
-		  x509.genkey extra_certificates signing_key.x509.keyid	\
-		  signing_key.x509.signer vmlinux-gdb.py \
+		  certs/signing_key.pem certs/signing_key.x509 \
+		  certs/x509.genkey \
+		  vmlinux-gdb.py \
 		  *.spec
 
 # Directories & files removed with 'make distclean'