Revert "Make QElapsedTimer default to invalid (and now non-POD)."

From the GitHub pull request's commit message:

On WebKit we use QElapsedTimer as implementation of the monotonically increasing timer, and we share values among processes, so we don't start the QElapsedTimer on purpose (so all processes keep the same reference).

Forcing us to start makes us have different timer references in each process, breaking animations.

https://codereview.qt-project.org/c/qt/qtbase/+/296548