[C++][Compute] Remove duplicate ThreadIndexer definition #29950
Description
#11350 moved ThreadIndexer to a header file to allow usage from the join node and incremented its capacity. We should have a single definition and the incremented capacity is a simple fix for nightly failures such as https://github.com/ursacomputing/crossbow/runs/3938882073#step:7:2363
Reporter: Ben Kietzman / @bkietz
Assignee: Ben Kietzman / @bkietz
PRs and other links:
Note: This issue was originally created as ARROW-14382. Please see the migration documentation for further details.