Remove vimdiff

While we're here:
- Remove references to the '+diff' feature, which has since been made
  non-optional.
- Update a few Vim instances with Nvim.

src/nvim/version.c

@@ -59,7 +59,6 @@ static char *(features[]) = {
   "+cursorbind",
   "+cursorshape",
   "+dialog_con",
-  "+diff",
   "+digraphs",
   "-dnd",
   "-ebcdic",