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

[Mehdi Amini via llvm-commits]

joker-eph 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.

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