Remove PSTL from the top-level monorepo (#143717)

The RFC for this removal can be found at:
https://discourse.llvm.org/t/rfc-removing-pstl/86807

Note, libc++ still supports PSTL. That support is integrated directly
into the libc++ source tree.

There is no release note for this removal because it's not really clear
that this was user-facing facilities or where such a release note should
live.

GitOrigin-RevId: f8ed45611b75ecac1d459aa6ae3b7051d435abe6
2 files changed