Remove vestigal conbench integration

[alamb]

Which issue does this PR close?

Part of #5504

Rationale for this change

The arrow monorepo https://github.com/apache/arrow has a nice automated system called conbench that tracks performance. At some point it was partially integrated into DataFusion as part of #1791 but that work was not completed
Now that @gruuya and @Dandandan are working on an alternate strategy (e.g. #9461) we can just get rid of the old vestigal code

What changes are included in this PR?

Remove conbench directory

Are these changes tested?

By CI

Are there any user-facing changes?

No, this is a developer only change

[gruuya]

While you're at it do you mind removing the two ./bench.sh run tpch_mem10 lines in the Benchmarks workflow (in relation to #9800 (comment))?

Here
https://github.com/apache/arrow-datafusion/blob/d7957636327fb8d89e6428152492107e39d614b2/.github/workflows/pr_benchmarks.yml#L50
and here
https://github.com/apache/arrow-datafusion/blob/d7957636327fb8d89e6428152492107e39d614b2/.github/workflows/pr_benchmarks.yml#L73

[alamb]

While you're at it do you mind removing the two ./bench.sh run tpch_mem10 lines in the Benchmarks workflow (in relation to #9800 (comment))?

#9856

[andygrove]

Thanks @alamb