scripts/coccinelle: drop bugon.cocci

The BUG_ON script was never safe, in that it was not able to check
whether the condition was side-effecting.  At this point, BUG_ON
should be well known, so it has probably outlived its usefuless.

Signed-off-by: Julia Lawall <Julia.Lawall@inria.fr>
Suggested-by: Matthew Wilcox <willy@infradead.org>