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ARROW-8784: [Rust] [DataFusion] Remove use of Arc from LogicalPlan

This PR replaces all uses of `Arc` with `Box` in the logical plan. It als removes a test that was still passing after this change but wasn't actually testing what it intended to test, and was therefore redundant.

Closes #7167 from andygrove/logical-plan-remove-arc

Authored-by: Andy Grove <andygrove@nvidia.com>
Signed-off-by: Andy Grove <andygrove73@gmail.com>