Revert "[llvm-exegesis] Add ability to assign perf counters to specific PID"

Revert "[llvm-exegesis] Add ability to assign perf counters to specific PID"

Revert "[llvm-exegesis] Introduce Subprocess Executor Mode"

This reverts commit 5e9173c43a9b97c8614e36d6f754317f731e71e9.
This reverts commit 4d618b52f6e05e41d35f56653cb36bf7d4dc794e.

Reverting the PID commit as it is currently breaking MinGW builds and
the way I'm checking for the presence of pid_t needs to be fixed and I
need to do some testing. The subprocess executor mode patch is a
dependent patch so also needs to be reverted and also needs some work as
it is currently failing tests where libpfm is installed and the kernel
version is less than 5.6.