Update Ballista to use new physical plan formatter utility

[andygrove]

Which issue does this PR close?

Closes #343.

Rationale for this change

See Closes #343.

What changes are included in this PR?

Remove Ballista's format_plan utility and change call sites to use displayable instead.

Are there any user-facing changes?

No, this is all internal debug logging.

[codecov-commenter]

Codecov Report

Merging #344 (727114d) into master (1702d6c) will increase coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #344      +/-   ##
==========================================
+ Coverage   75.72%   75.73%   +0.01%     
==========================================
  Files         143      143              
  Lines       23832    23776      -56     
==========================================
- Hits        18046    18007      -39     
+ Misses       5786     5769      -17     

Impacted Files	Coverage Δ
ballista/rust/core/src/utils.rs	0.00% <ø> (-30.44%)	⬇️
ballista/rust/executor/src/flight_service.rs	0.00% <0.00%> (ø)
ballista/rust/scheduler/src/planner.rs	69.46% <100.00%> (ø)
...lista/rust/core/src/execution_plans/query_stage.rs	42.85% <0.00%> (-4.77%)	⬇️
datafusion/src/logical_plan/operators.rs	77.14% <0.00%> (-2.86%)	⬇️
datafusion/src/physical_plan/expressions/sum.rs	78.20% <0.00%> (+1.28%)	⬆️
datafusion/src/physical_plan/mod.rs	87.05% <0.00%> (+2.35%)	⬆️
datafusion/src/physical_plan/expressions/mod.rs	71.42% <0.00%> (+3.57%)	⬆️

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[alamb]

Looks like an improvement to me :)