Disable the 'Replace' affordance on generic template parts

[jameskoster]

The 'replace' affordance on template parts makes it easy to quickly swap out one header (or footer) for another:

It's much less useful when the template part has no assigned area. For example in my theme I have a content.html template part that I use in various query loops. When I click 'replace' in the editor I'm presented with all existing template parts, including things like headers and footers:

That "Choose a general" heading in the modal is especially strange :D

I propose that we disable this functionality altogether for generic template parts. It just seems very difficult to populate the modal with relevant suggestions when there is no semantic area assigned to the template part. I would never replace my content.html template part with header.html.