What to do when you publish a paper that accidentally included a wrong or irrelevant citation?
Plagiarism is the use of another author's ideas or words without proper credit.  You haven't done that, so there's no need to worry about it.  Also, people don't retract papers over trivial editing errors like this.

However, since it's something that might be confusing to a reader, it may be worth asking the journal about printing a correction.  Simply get in touch with the editor who originally handled your paper, or if they no longer work with the journal, contact the editor-in-chief.  They would typically publish a one-sentence note in some future issue of the journal, stating that the references were included by mistake and should be ignored.  Alternatively, they may decide the matter is too trivial to be worth the space to correct it.

Either way, you should post the correction on your web page, and any other place where the paper is publicly available (preprint servers, etc).  

This is no big deal and happens all the time.  Just get it fixed, move on, and be more careful next time.