Update references.bib

Author: marceltaeumel

_research/references.bib

@@ -1505,7 +1505,6 @@ @inproceedings{bergel_userchangeable_2007
 @article{bergel_classboxes_2005,
   title = {Classboxes: Controlling Visibility of Class Extensions},
   volume = {31},
-  url = {https://www.esug.org/data/ESUG2004/iam-04-008.pdf\#page=186},
   doi = {10.1016/j.cl.2004.11.002},
   number = {3-4},
   journal = {Computer Languages, Systems and Structures},