gh-138013: Remove load_tests in test_io.test_general (#138771)

Rely on default test discovery.

Validation:
```bash
 # Run before commit
./python -m test test_io -uall,walltime,largefile,cpu,extralargefile -M25G -o --fail-env-changed -j0 --list-cases | sort > old_cases.txt
 # Run after commit
./python -m test test_io -uall,walltime,largefile,cpu,extralargefile -M25G -o --fail-env-changed -j0 --list-cases | sort > new_cases.txt

diff new_cases.txt old_cases.
 # <outputs no changes in case list>
```