I wouldn't normally answer a question that already has 16 answers, but all the other answers are wrong, and the right answer is so simple. The question says, "Is there a simple way to delete all tracking branches whose remote equivalent no longer exists?"
If "simple" means deleting them all in one go, not fragile, not dangerous, and without reliance on tools that not all readers will have, then the right answer is: no.
Some answers are simple, but they don't do what was asked. Others do what was asked, but are not simple: all rely on parsing Git output through text-manipulation commands or scripting languages, which may not be present on every system. On top of that, most of the suggestions use porcelain commands, whose output is not designed to be parsed by script ("porcelain" refers to the commands intended for human operation; scripts should use the lower-level "plumbing" commands).
Further reading:
If you want to do this safely, for the use case in the question (garbage-collect tracking branches which have been deleted on the server but still exist as local branches) and with high-level Git commands only, you have to
git fetch --prune (or git fetch -p, which is an alias, or git prune remote origin which does the same thing without fetching, and is probably not what you want most of the time). - Note any remote branches that are reported as deleted. Or, to find them later on,
git branch -v (any orphaned tracking branch will be marked "[gone]"). git branch -d [branch_name] on each orphaned tracking branch
(which is what some of the other answers propose).
If you want to script a solution, then for-each-ref is your starting point, as in Mark Longair's answer here and this answer to another question, but I can't see a way to exploit it without writing a shell script loop, or using xargs or something.
Background explanation
To understand what's happening, you need to appreciate that, in the situation of tracking branches, you have not one branch, but three. (And recall that "branch" means simply a pointer to a commit.)
Given a tracking branch feature/X, the remote repository (server) will have this branch and call it feature/X. Your local repository has a branch remotes/origin/feature/X which means, "This is what the remote told me its feature/X branch was, last time we talked," and finally, the local repository has a branch feature/X which points to your latest commit, and is configured to "track" remotes/origin/feature/X, meaning that you can pull and push to keep them aligned.
At some point, someone has deleted the feature/X on the remote. From that moment, you are left with your local feature/X (which you probably don't want any more, since work on feature X is presumably finished), and your remotes/origin/feature/X which is certainly useless because its only purpose was to remember the state of the server's branch.
And Git will let you automatically clean up the redundant remotes/origin/feature/X -- that's what git fetch --prune does -- but for some reason, it doesn't let you automatically delete your own feature/X... even though your feature/X still contains the orphaned tracking information, so it has the information to identify former tracking branches that have been fully merged. (After all, it can give you the information that lets you do the operation by hand yourself.)
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