Extended paper - should I keep someone who virtually had no contribution? I had a paper accepted at a conference and want to extend it with some new material based on the comments and submit it to a journal.

One of the co-authors was added to the conference paper because he was the head of the group where I was working. His contribution were textual suggestions to the manuscript. We have since split ways and he has no influence whatsoever in the extension. Again, he had little influence in the conference version and I added him mostly to avoid conflict, since these days people expect co-authorship based on their position.

Since he did not contribute neither to the conference version nor to the journal extension (and he has no position of authority over me anymore), I do not want to add him to the list of authors in the extended journal version. How acceptable/unacceptable is this?
In all journals (CS/AI) I’ve worked with you need all authors of the conference paper(s) on which the journal version is based on to agree on the author list in the journal version. Some editors will just not allow a paper to be accepted before explicit consent has been obtained. 

It is also unfair to an extent towards this professor. Think about this professor’s perspective: they may believe their contribution was much greater than just some textual edits, and once they realize that you submitted a version without their knowledge or consent, they’ll just email the editor and the paper will not go through to publication before it’s all sorted out.

I suggest you email the professor, tell them your plan, and whether they’d like to still be written as a coauthor. Most people would get the hint. Even if they’d still want to be included then there’s not much you can do about it: you should have established authorship in the conference version.