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GH-45219: [C++][Examples] Update examples to disable mimalloc (#45220)



### Rationale for this change

I think the intention was to disable the default allocator which used to be jemalloc. However since #44951 we have switched the default allocator from jemalloc to mimalloc. We should update the examples.

### What changes are included in this PR?

Change disabling jemalloc to disabling mimalloc for examples.

### Are these changes tested?

Manually tested.

### Are there any user-facing changes?

None.

* GitHub Issue: #45219

Authored-by: Rossi Sun <zanmato1984@gmail.com>
Signed-off-by: Rossi Sun <zanmato1984@gmail.com>