How should I respond to a reviewer's complaint about self-citation?
The answers here of Solar Mike and corey979 are correct, but let me point out two additional issues. 

First, it might be that without your citations you could validly be accused of self plagiarism. Readers of the current work need to be able to trace back the ideas to earlier work. This is why we cite ourselves rather than just reuse old work. 

Second, if you want a mental check on whether a self-citation is proper or not, just ask whether you would still be giving this citation if the other paper were written by someone unknown to you. If the answer is yes, then it is certainly proper - even essential - to cite it. If the answer is no, then you should probably forgo. 

As for a response, you could simply ignore it. However, if you think that it is affecting the editor's decision, you could point out the self-plagiarism aspect.