lib/deltas: Move variant read helper into the deltas code

What the deltas code is doing is weird/unfortunate.  The name
`ot_variant_read()` conflicts too much with `ot_variant_read_fd()`.
Since nothing else uses it, move it into the deltas code.

Closes: #1254
Approved by: jlebon

src/libotutil/ot-variant-utils.h

@@ -39,8 +39,6 @@ gboolean ot_variant_read_fd (int                  fd,
                              GVariant           **out_variant,
                              GError             **error);
 
-GInputStream *ot_variant_read (GVariant             *variant);
-
 GVariantBuilder *ot_util_variant_builder_from_variant (GVariant            *variant,
                                                        const GVariantType  *type);