Remove old benchmark infra and replace with package-benchmark

[ktoso]

Remove old benchmark infra which causes sendability warnings.

There's proper benchmark packages now and we should use them instead, depend on: https://github.com/ordo-one/package-benchmark.git

Modifications: Remove custom benchmark infra. Adopt existing benchmark package -- this can also nicely track allocations.

Result:

No sendability warnings; less deprecated benchmark infra

[ktoso]

@swift-ci please smoke test