[vim/.vimrc]: Switch back to traditional <C-f>,<C-b>,etc. scroll maps

I still think that the earlier mappings were more intuitive, as the
<C-f>/<C-d> parallel to j/k was beautiful. However, since I'm using
`less` and other utilities that stick with the traditional maps, and
because others sometimes use my vim as well, I'm switching back (at
least, temporarily).