Also remove the require-cachable.js benchmarks because now all builtin
modules are cacheable, it would be comparing oranges to apples when
we try to compare the performance of loading all cacheable modules
in different Node.js binaries since the set of modules are just
different. Comparison of startup performance that involves loading
of the long-standing, stable builtins is already covered by the
require-builtins benchmark.

The previous benchmark was inaccurate in several ways:

- It previously measured "how many workers/child processes can finish
  loading in parallel in 1 second" which is prone to wrong
  interpretations. It's also not typical for users to spin up >20
  workers/child processes in parallel, which was what the benchmark
  usually ended up doing. The parallelism can also be greatly affected
  by system configurations.
- The time spent on loading a Node.js binary for the first time varies
  greatly depending on system configurations, but it was inevitable
  for the benchmark to load Node.js multiple times to reduce the
  influence of fluctuations.

This patch rewrites the benchmark so that:

- It now measures "how long it takes to finish 30 workers/child processes
  in serial" which is generally more in line with how other benchmarks
  are written and with the figures one gets from doing something like
  `time node index.js` or `hyperfine 'node index.js'`
- It now warms up the loading of the Node.js instance to reduce the
  statistical outliers, so that we get more meaningful figures when
  trying to compare the startup performance of different Node.js
  binaries.

…mark