[CLEANUP] Code cleanup in ext:reports

* Use getter for $GLOBALS
* Use use-statements to avoid fully qualified class names
* Add missing @return annotations

Change-Id: Ie49fc42a33677a0de451becf12c06fe883dac8a2
Resolves: #72513
Releases: master
Reviewed-on: https://review.typo3.org/45559


Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>