rust: stop claiming the C index is compatible with the rust code

This is no longer the case since the introduction of the pure Rust Index, and
was probably not the case since the MixedIndex itself.

So we fix the dedicated attribute value.

mercurial/cext/revlog.c

@@ -3226,7 +3226,6 b' static int index_init(indexObject *self,'
 	}
 
 	if (self->format_version == format_v1) {
-		self->rust_ext_compat = 1;
 		self->entry_size = v1_entry_size;
 	} else if (self->format_version == format_v2) {
 		self->entry_size = v2_entry_size;