lib/sign: public API optimisation

Removed from public `ostree_sign_detached_metadata_append` function.
Renamed `metadata_verify` into `data_verify` to fit to real
functionality.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>

src/libostree/ostree-repo-pull.c

@@ -1586,7 +1586,7 @@ ostree_verify_unwritten_commit (OtPullData                 *pull_data,
             }
 
           /* Set return to true if any sign fit */
-          if (ostree_sign_metadata_verify (sign,
+          if (ostree_sign_data_verify (sign,
                                            signed_data,
                                            signatures,
                                            &local_error