Remove OSS-Fuzz from third_party

The only code that uses it is the benchmark integration script.
That script needs to switch versions of OSS-Fuzz anyway, so there's not really a point to having an OSS-Fuzz checkout that is pinned to a particular revision.
The downside of having it is annoyance. When it gets updated, doing git pull --rebase isn't sufficient to update the version and many PRs have accidentally tried to update it. It also adds a trivial but unneeded step to setting up FuzzBench.