Remove small patch exception checking in hack/make/validate-dco

I propose allowing maintainers to add a DCO when its missing from the git commit. 

Either by asking the commiter to add it to the Pull Request's comments, or by deciding its a trivial change, so can be signed by them.

replaces #9010 

#9079 is an example of this in action.

I think it's fine for now as it's what tianon wanted