lib: Drop GFile variant mapping API for fd-relative

In addition to generic fd relative porting,
this is a necessary preparatory step for libglnx porting, because
when I tried to use `g_mapped_file_new` I hit an issue with
it using a different error domain from GIO.

Thankfully libglnx consistently uses the GIO error domain, and here
we're now using it for the `open()` call.

Closes: #317
Approved by: jlebon

src/libotutil/ot-variant-utils.h

@@ -42,12 +42,6 @@ gboolean ot_util_variant_save (GFile *dest,
                                GCancellable *cancellable,
                                GError  **error);
 
-gboolean ot_util_variant_map (GFile *src,
-                              const GVariantType *type,
-                              gboolean trusted,
-                              GVariant **out_variant,
-                              GError  **error);
-
 gboolean ot_util_variant_map_at (int dfd,
                                  const char *path,
                                  const GVariantType *type,