Merge pull request #1896 from alalek:docs_fix_bib_Kroeger2016

docs: fix Kroeger2016 bib reference

move from opencv_contrib

Author: alalek

modules/optflow/doc/optflow.bib

@@ -38,13 +38,6 @@ @inproceedings{Weinzaepfel2013
   organization={IEEE}
 }
 
-@inproceedings{Kroeger2016,
-  author={Till Kroeger and Radu Timofte and Dengxin Dai and Luc Van Gool},
-  title={Fast Optical Flow using Dense Inverse Search},
-  booktitle={Proceedings of the European Conference on Computer Vision ({ECCV})},
-  year = {2016}
-}
-
 @inproceedings{Brox2004,
   title={High accuracy optical flow estimation based on a theory for warping},
   author={Brox, Thomas and Bruhn, Andr{\'e}s and Papenberg, Nils and Weickert, Joachim},