real: remove PolyGP

The benchmark doesn't compile and was not enabled. I tried fixing it,
but it seems to take excessive amount of time & memory (didn't finish
in 60s, which required over 10GiB of RAM). Sounds like another
candidate for removal.

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

Test Plan: build & run

Reviewers: bgamari

Differential Revision: https://phabricator.haskell.org/D3329