[TASK] Use __DIR__ instead of dirname(__FILE__)

Supported since PHP 5.3.0.
Fluid from Flow uses __DIR__ and the backporter
used a replacement to transform it to
dirname(__FILE__).

See also patch for backporter:
https://review.typo3.org/19955

Resolves: #47206
Releases: 6.0, 6.1
Change-Id: If32852e74625990b6323848074fe54fc5e1fb554
Reviewed-on: https://review.typo3.org/19956
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring