[C++] Remove ExecBatchIterator and usages thereof

[asfimport]

The only place left using it is in GroupBy in arrow/compute/exec/aggregate.cc. This can be refactored to use ExecSpan.

As part of this removal, we should adapt the benchmarks for ExecSpanIterator to demonstrate the performance improvement there

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

Related issues:

• [C++] Improve array expression and kernel evaluation performance on small inputs (is a child of)

_{Note: This issue was originally created as ARROW-16929. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
Resolved in a related PR