[CLEANUP] Cleanup backend toolbar item registration

* Remove side effects from class file
* Move registration script to Resources/PHP folder

Resolves: #62775
Related: #61467
Releases: master
Change-Id: I3a924825534e7b02e30c95b635375e2158f33a02
Reviewed-on: http://review.typo3.org/33895
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benjamin Mack <benni@typo3.org>
Tested-by: Benjamin Mack <benni@typo3.org>