ARROW-14382: [C++][Compute] Remove duplicated ThreadIndexer definition
ARROW-14382: [C++][Compute] Remove duplicated ThreadIndexer definition

https://github.com/apache/arrow/pull/11350 moved ThreadIndexer from `aggregate_node.cc` to `compute/exec/util.h` to allow usage from the join node and incremented its capacity, but did not remove the original implementation.

Closes #11463 from bkietz/remove-duplicated-ThreadIndexer

Authored-by: Benjamin Kietzman <be26f8d5d161e4d7965ee00602589fb2b74149f2@gmail.com>
Signed-off-by: David Li <dc24f9cfc84fb1675ce65c8444a2c3e2f0e5f297@gmail.com>

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