No need to kill process that already exited.

This code was broken by this commit https://github.com/VioletGiraffe/cppcheck-vs-addin/commit/615a484860dff7e014fa1d90b39ecad961cc365c#diff-b7edabae74f3f333dd2aa71e3f063a2dL122 Note `process = null` line removed for no reason.