core: Remove ostree_checksum_update_meta that was not implemented

If a symbol falls in a git merkle tree forest, but no one hears it,
did it ever exist?

I'm claiming this isn't an API/ABI break because nothing could
have actually used this.  `git log -S checksum_update_meta` leads
me to 38ef75e6e0 which was before
we really had a stable shared library.

Closes: #263
Approved by: giuseppe

src/libostree/ostree-core.h

@@ -214,9 +214,6 @@ gboolean ostree_parse_refspec (const char *refspec,
                                char      **out_ref,
                                GError    **error);
 
-_OSTREE_PUBLIC
-void ostree_checksum_update_meta (GChecksum *checksum, GFileInfo *file_info, GVariant  *xattrs);
-
 _OSTREE_PUBLIC
 const char * ostree_object_type_to_string (OstreeObjectType objtype);