remove sciencedirect link

docs/source/refs.bib

@@ -32,7 +32,6 @@ @article{ZALESAK1979335
 year = "1979",
 issn = "0021-9991",
 doi = "https://doi.org/10.1016/0021-9991(79)90051-2",
-url = "http://www.sciencedirect.com/science/article/pii/0021999179900512",
 author = "Steven T Zalesak",
 abstract = "The theory of flux-corrected transport (FCT) developed by Boris and Book [J. Comput. Phys. 11 (1973) 38; 18 (1975) 248; 20 (1976) 397] is placed in a simple, generalized format, and a new algorithm for implementing the critical flux limiting stage' in multidimensions without resort to time splitting is presented. The new flux limiting algorithm allows the use of FCT techniques in multidimensional fluid problems for which time splitting would produce unacceptable numerical results, such as those involving incompressible or nearly incompressible flow fields. The “clipping” problem associated with the original one dimensional flux limiter is also eliminated or alleviated. Test results and applications to a two dimensional fluid plasma problem are presented."
 }