Vim: use visual mode mappings instead of visual+select

I never explicitly enter select mode, and even if I did, these mappings
wouldn't be appropriate for it (generally, you'd want to avoid any
mappings involving printable characters in select mode, because those
keys should replace the currently selected text).

Note there are still a couple of "v" (ie. visual + select) mappings in
my term.vim plugin file, but those are actually appropriate (not
printable, and we want them to work in literally all modes) so I've left
those as-is.