remove knovel (#3338)

Elsevier acquired Knovel in 2013

`data/knovel` has only one domain: `knovel.com`, and it's already in
`data/elsevier`

data/category-scholar-!cn

@@ -8,7 +8,6 @@ include:doi
 include:elsevier
 include:google-scholar
 include:ieee
-include:knovel
 include:mit
 include:proquest
 include:sci-hub