Remove custom GC configuration

During some non-real-world benchmarking (single-node cluster, `fsync`
disabled, running in `tmpfs`) it turned out the custom GC configuration
as set up by Arakoon has a non-negligible performance impact: the `set`
scenario of the built-in benchmark goes from +- 115s to +- 93s.

It looks like the default GC settings (of OCaml 4.01.0) are better
suited for the Arakoon workload.

Next to this, the custom settings removed the ability to set
site-specific configurations using `OCAMLRUNPARAM`.