Remove vendored dependency

[panglesd]

Remove vendored ppx_sexp_conv and add it as normal dependency for ppxlib-bench.

[avsm]

Yes please! The vendoring here also breaks the build of ppxlib in a bigger dune monorepo (where it'll conflict with another version of ppx_sexp_conv elsewhere).

[pitag-ha]

Thanks, @panglesd, for the quick PR! To be clear: this breakage is on me.

Notice that if we treat the benchmark PPX as a normal dependency, we'll benchmark some ppxlib release (as opposed to its dev version). It's possible that we could find a way around that, but @kit-ty-kate has already found a different great solution to the problem: #386. It's already merged and patch released.