ARROW-4586: [Rust] Remove arrow/mod.rs as it is not needed

`mod.rs` is not needed as `lib.rs` imports sub-modules directly.  In fact, it's not compiled at all from what I can see...

Author: Paddy Horan <[email protected]>

Closes #3659 from paddyhoran/remove-mod and squashes the following commits:

513eaa4 <Paddy Horan> Removed `arrow/mod.rs`

Author: paddyhoran