[taskmanager] Simplifiy one of the two PID lookups

Summary:
KwindowInfo already has the PID, which we use in another case where PID is used.

More importantly I have a patch coming to KWindowSystem that returns the correct PID
for processes inside a PID namespace, but I don't alter the NETProperties.

Reviewers: #plasma, apol, hein

Reviewed By: #plasma, apol, hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22753