[llvm] [pstl] Remove PSTL from the top-level monorepo (PR #143717)

[Aaron Ballman via llvm-commits]

AaronBallman wrote:

> I'm concerned this requires a master buildbot restart, if you don't manage to get @gkistanova acknowledge this quickly we need to revert or land another fix in the monorepo in the meantime.

We can certainly revert (the churn would be unfortunate though). I'm not certain what other fix would be possible if a restart is required.

https://github.com/llvm/llvm-project/pull/143717