Allow DeleteBehavior to be reset to the default

[ajcvickers]

Breaking out of #12661

For example, by allowing null to be set in the fluent API.

Note that the value in core metadata is not nullable, so this means either making it nullable as well (which would be a more significant break) or causing null in the fluent API to re-run the convention logic used if no value has been set.

[AndriySvyryd]

Note that setting it to null will just reset the value to the model default, it won't rerun the conventions that might have modified it. To achieve that we would either need default value conventions or metadata layering (part of #214) where higher configuration source would not remove the lower source value, but just hide it.