Remove gnuplot dependency for benchmarks

[punchagan]

gnuplot was added as a dependecy as a part of the PR #343 since a library for running the Irmin benchmarks depended on it. But, we don't currently run the irmin benchmarks and the gnuplot dependency installs a whole bunch of X11 dependencies when setting up the OS for running the benchmarks. This makes the setup longer than it needs to be, as of now.

[shakthimaan]

we don't currently run the irmin benchmarks

My suggestion is to leave a TODO comment with the actual dependency name instead of removing it completely. When the ppxlib dependency is resolved, we will run the Irmin benchmarks again.

[shakthimaan]

Thanks! LGTM.