processhelp.pm: improve `taskkill` calls (Windows)

- drop `tasklist` call before `taskkill`.
  `taskkill` offers two ways to kill a `pid`:
  1. `-pid <pid>`
     If `<pid>` is missing it returns 128 and outputs:
     ```
     ERROR: The process "<pid>" not found.
     ```
  2. `-fi "PID eq <pid>"`
     If `<pid>` is missing, it returns 0 and outputs:
     ```
     INFO: No tasks running with the specified criteria.
     ```
  The curl runner script doesn't check the result of the call and both
  stdout and stderr are redirected to NUL.
  Meaning the `tasklist` calls pre-verifying if the PID exists are not
  necessary and we can drop them to put less strain on the runner
  environment.

- log a `taskkill` call missed earlier.
  Follow-up to e53523fef07894991c69d907a7c7794c7ada4ff4 #14859

- streamline `taskkill` calls by using the `-pid` option
  (was `-fi <filter-expression>`).

- make `taskkill` in `pidterm()` use `-t` to kill the process tree.

Ref: #11009
Closes #14959

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