disable logging to kscreen.log by default

Summary:
Most of the issues that this feature should uncover have been uncovered,
it makes sense to disable the logging (and futzing around with
QCategorizedLogging) by default, and only use it to debug specific
setups.

Test Plan: Adjusted autotests accordingly.

Reviewers: #plasma, mart

Reviewed By: mart

Subscribers: graesslin, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D3993

BUG:361688
CHANGELOG:Disable logging to kscreen.log by default, re-enable with export KSCREEN_LOGGING=1

cherry-picked from cea4c63f