[TASK] Use magic __CLASS__ in getInstance()-methods

Some classes contain static getInstance()-methods that return
an instance of themselves. Instead of using the correct namespace
and classname everywhere, just use __CLASS__.

Change-Id: I22770505acded0c54b07dd0e239976ad01fdf893
Resolves: #50615
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/22887
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters