Fixed #23444 -- Prevented unnecessary ContentType object creation when using inlines

ContentType object is mistakenly created for `through` model of M2M relation when it is used in inline.
I'm not sure re-assigning `InlineAdminForm.__class__.original_content_type_id` this is thread safe.

What about defining it as a cached property that raise the warning above and returns `_original_content_type_id` which is assigned in this conditional.