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ARROW-7256: [C++] Remove ARROW_MEMORY_POOL_DEFAULT macro

This should fix the doxygen warning (which is turned into error).

Closes #6877 from kszucs/ARROW-7256

Authored-by: Krisztián Szűcs <szucs.krisztian@gmail.com>
Signed-off-by: Krisztián Szűcs <szucs.krisztian@gmail.com>