Process: Revert "Add function to get parent PID"

Revert !322 (merged), except for the incidental undef kwsysProcess_KillPID fix.

We've found another approach for the motivating use case. See cmake/cmake!10230 (comment 1618816).

assigned to @brad.king

mentioned in merge request !322 (merged)

Do: merge

mentioned in commit 6f535da1

merged manually

mentioned in merge request cmake/cmake!10230 (merged)