Remove fibon

Summary:
It doesn't compile, nobody is running it and upstream seems to have
abandoned it too. IMHO, at this point we should simply remove it and
focus on improving the benchmarks that do work & adding new ones.

See also the discussion in #11501

Signed-off-by: Michal Terepeta <michal.terepeta@gmail.com>

Test Plan: compile & run nofib

Reviewers: bgamari

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