What do I do if somebody asks me to join his accusation of my colleague of academic misconduct?

Delete the email (or move it to one of your folders where you keep old ones).

From what you describe, you have no business joining the complaint since you have no complaint again colleague A. It is, in other words, not something that actually affects you: someone has beef with colleague A, and that is their business, nor yours. Worse, if you did get involved -- even if just by writing back to B --, there is nothing for you to gain, just collegiality to lose. It's neither worth your time nor your reputation to act any further than reading, then ignoring the email.

If anyone ever asks you whether you got the email, just tersely say "Yes, I got it; it was none of my business, so I ignored it."

Should I answer Prof. B that this way to address the issue was not ok?

You can, if you feel strongly about it. But I think it is perfectly okay not to respond. First of all, you are one of a long list of recipients, so it would be quite unreasonable for Prof. B to expect each and every recipient of the email to set aside their work and life to dedicate themselves to studying in detail the situation he is describing and then write an equally detailed reply. Basically, to put it bluntly, Prof. B's email can be reasonably classified as spam and there is certainly no obligation to reply to it in any way.

Second of all, you are on sabbatical, which is a sacred time you should devote to nurturing your own interests and passions and ignoring the daily trivia and tedium of academic life. The case for not responding is therefore even stronger (to the extent it needed any strengthening).

Should I inform Prof. A that I found this email inappropriate?

Again, you have been implicated in this business completely through the actions of others and have no duty to take any action. If you feel strongly enough about the inappropriate email that you feel compelled to write to Prof. A, go ahead. But there is absolutely no need to do so.

Should I just do as if I haven't read this email?

I wouldn't falsely pretend that I haven't read the email, if that's what you mean - that would be dishonest and unprofessional (and besides, you have now documented publicly your knowledge of the email ;-)). Nor would I plan on saying (as Wolfgang Bangerth suggests in his answer) that I deleted it since it was "none of my business" (I don't think that unethical behavior by others is none of my business - it would be perfectly appropriate to consider it my business if I were so inclined). But as I said it would be completely appropriate to file the email away (or delete it, as you prefer) and take no further action. Whether you should do that depends on whether you are interested in getting involved in the story.

If you have knowledge, independently of B's accusation, that A is behaving badly, it's up to you to join B or not, depending on how much you care. If you don't have any such knowledge then you shouldn't.

If you have knowledge that B's accusations are wrong then you should inform B if you think B is mistaken, and you should inform A if you think B is intentionally spreading false accusations. If you have no knowledge that B's accusations are wrong then you may contact A about it or not, depending on how much you care.

Consider that getting involved in any way may cause you inconvenience, and not getting involved when someone is wronged may cause you a bad conscience (or worse).