Remove variables declared as unused

They are now always being used.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/library/psa_crypto_aead.c b/library/psa_crypto_aead.c
index 337748a..aa266ea 100644
--- a/library/psa_crypto_aead.c
+++ b/library/psa_crypto_aead.c
@@ -473,9 +473,7 @@
     else
 #endif /* MBEDTLS_PSA_BUILTIN_ALG_CHACHA20_POLY1305 */
     {
-        ( void ) operation;
         ( void ) nonce;
-        ( void ) nonce_length;
 
         return ( PSA_ERROR_NOT_SUPPORTED );
     }