OstreeGpgVerifier: Take the signed data as a GBytes

Similar to c2b01ad.  For some reason I was thinking the commit data
still needed to be written to disk prior to verifying, but it's just
another artifact of spawning gpgv2 (predates using GPGME).

Makes for a nice cleanup in fetch_metadata_to_verify_delta_superblock()
as well.

src/libostree/ostree-gpg-verifier.h

@@ -42,7 +42,7 @@ OstreeGpgVerifier *_ostree_gpg_verifier_new (GCancellable   *cancellable,
                                              GError        **error);
 
 gboolean      _ostree_gpg_verifier_check_signature (OstreeGpgVerifier *self,
-                                                    GFile             *file,
+                                                    GBytes            *signed_data,
                                                     GBytes            *signatures,
                                                     gboolean          *had_valid_signature,
                                                     GCancellable      *cancellable,