sandboxfs: remove

sandboxfs was an experiment to increase sandboxing performance in bazel,
but it never reached a stable release.

The author of sandboxfs left Google in 2020 and there have been no
updates to it since then.

bazel dropped sandboxfs in the bazel 7 release. To quote their release
notes:

    The sandboxfs sandboxing strategy is removed. It hadn't been
    maintained for a long time, it didn't work for most users and it was
    not consistently faster while being complex to set up. sandboxfs
    performance is heavily dependent on the specific setup (setup costs
    are lower, but you have to pay a penalty for the use of each input)
    and there are scenarios where it is faster and scenarios where it is
    slower. Overall it is not worth its weight.

    - https://github.com/bazelbuild/bazel/commit/217fafe2b492de7349547cc3be2b9eef38628055

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