Remove start_process helper method from bootstrap

[Shourya742]

This PR removes start_process utility method.

r? @Kobzol

[Kobzol]

Thanks! Could you please do a small benchmark to check how long it takes to run all the gitinfos now? Before, these three commands executed in parallel, while now they run sequentially. I don't expect a lot of changes, but we do invoke a bunch of gitinfos, so it would be nice to check.

[bors]

☔ The latest upstream changes (presumably #142483) made this pull request unmergeable. Please resolve the merge conflicts.

[the8472]

I itentionally added this as a perf optimization in #131954, is there a problem with this method existing?

[Kobzol]

We're trying to get rid of various adhoc ways of executing commands in bootstrap, and instead use a unified API so that we can get caching/tracing/mocking.

We already noticed that this helps perf. a bit, yeah. We'll add some API to allow batching commands to keep the current behavior. In fact, I think we could parallelize across all GitInfo invocations, we do a lot of them.

[the8472]

Ok, the PR didn't provide any context, so it looked like a change without much benefit.

[Kobzol]

Obsoleted by #142591.