Fix double return statement in cipher.c

This was introduced in ce1ddee13a171
diff --git a/library/cipher.c b/library/cipher.c
index 16037fb..5d7e53f 100644
--- a/library/cipher.c
+++ b/library/cipher.c
@@ -1100,8 +1100,6 @@
          * operations, we currently don't make it
          * accessible through the cipher layer. */
         return( MBEDTLS_ERR_CIPHER_FEATURE_UNAVAILABLE );
-
-        return( 0 );
     }
 #endif /* MBEDTLS_USE_PSA_CRYPTO */