Hide Priority override when globally disabled

A node type can be prevented from appearing in the sitemap by setting its priority to -1 at the XML Sitemap settings page (admin/settings/gsitemap). However, I am still given the opportunity to override the priority when creating a new node, which doesn't make sense. The expected behavior for the "Priority override" form field is to not show up when it has been disabled in the global settings.

The attached patch fixes this by checking the node type's global priority setting before altering the node's form.