[Rust] Remove arrow/mod.rs as it is not needed

[asfimport]

Reporter: Paddy Horan / @paddyhoran
Assignee: Paddy Horan / @paddyhoran

PRs and other links:

• GitHub Pull Request #3659

_{Note: This issue was originally created as ARROW-4586. Please see the migration documentation for further details.}

[asfimport]

Andy Grove / @andygrove:
Issue resolved by pull request 3659
#3659