remove unused SPLIT_CHAR

`SPLIT_CHAR` is not used any more:

```
(00:31:05) [robert@tequila-apple] ~/npm (master) $ git grep SPLIT_CHAR
lib/npm.js:require("path").SPLIT_CHAR = process.platform === "win32" ? "\\" : "/"
```

If it would be used, `require("path").sep` would replace it now.