How to tell a faculty member that I don't need him on my thesis committee anymore?
I think you're making a bigger deal about this than necessary.  You enlisted Professor A to serve on your committee because you needed a fifth man, not because of any close research connection.  More recently, but still before signing any paperwork, you found Professor B, who does have a close research connection.

Just tell Professor A that!  Problem solved.  You don't have to dither about how to present this information: send it in an email or send it in person.  The information of the previous paragraph is sufficient: you don't need to dwell on your connection with Professor A as much as you did in your question.  (But by the way: asking about your progress at the copy machine is what I would call "friendly".  At least he knows who you are and something about your progress in the program, and he cares enough to ask about it sometimes.  That puts him ahead of the curve in many academic departments at many universities.)  

What you do not seem to realize is that if Professor A has no close connection with your work, it is overwhelmingly likely that he was being friendly indeed by volunteering to serve on your committee anyway, and he'll be equally happy or, more probably, a little happier not to serve.  Since he is a senior tenured professor he has lots of stuff to do.  Serving as the fifth man on a student's committee is not nearly such a prestige job that his layoff needs to be sugarcoated.  He'll understand, and he'll be especially happy that someone with more relevant expertise will be taking his place.