### What changes were proposed in this pull request?

This PR removes the manual port of `heapq3.py` introduced from SPARK-3073. The main reason of this was to support Python 2.6 and 2.7 because Python 2's `heapq.merge()` doesn't not support `key` and `reverse`.

See
- https://docs.python.org/2/library/heapq.html#heapq.merge in Python 2
- https://docs.python.org/3.8/library/heapq.html#heapq.merge in Python 3

Since we dropped the Python 2 at SPARK-32138, we can remove this away.

### Why are the changes needed?

To remove unnecessary codes. Also, we can leverage bug fixes made in Python 3.x at `heapq`.

### Does this PR introduce _any_ user-facing change?

No, dev-only.

### How was this patch tested?

Existing tests should cover. I locally ran and verified:

```bash
./python/run-tests --python-executable=python3 --testname="pyspark.tests.test_shuffle"
./python/run-tests --python-executable=python3 --testname="pyspark.shuffle ExternalSorter"
./python/run-tests --python-executable=python3 --testname="pyspark.tests.test_rdd RDDTests.test_external_group_by_key"
```

Closes #29229 from HyukjinKwon/SPARK-32435.
Authored-by: NHyukjinKwon <gurwls223@apache.org>
Signed-off-by: NHyukjinKwon <gurwls223@apache.org>