integrity: Remove references to module keyring

From what I can tell, it has never been used.

Mimi: This was introduced prior to Rusty's decision to use appended
signatures for kernel modules.

Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Acked-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: James Morris <james.morris@microsoft.com>

security/integrity/digsig.c

@@ -34,7 +34,6 @@ static const char * const keyring_name[INTEGRITY_KEYRING_MAX] = {
 	".evm",
 	".ima",
 #endif
-	"_module",
 	".platform",
 };