Remove the explicit IO platform setting (#28569)

tools/internal_ci/linux/aws/grpc_run_basictests_python_aarch64.sh

@@ -29,9 +29,7 @@ cd grpc
 git submodule update --init
 
 # build and test python (currently we only test with python3.6, but that's ok since our aarch64 testing resources are limited)
-tools/run_tests/run_tests.py -l python --compiler python3.6 -c opt --iomgr_platform native -t -x run_tests/python_linux_opt_native/sponge_log.xml --report_suite_name python_linux_opt_native --report_multi_target || FAILED=true
-tools/run_tests/run_tests.py -l python --compiler python3.6 -c opt --iomgr_platform asyncio -t -x run_tests/python_linux_opt_asyncio/sponge_log.xml --report_suite_name python_linux_opt_asyncio --report_multi_target || FAILED=true
-tools/run_tests/run_tests.py -l python --compiler python3.6 -c opt --iomgr_platform gevent -t -x run_tests/python_linux_opt_gevent/sponge_log.xml --report_suite_name python_linux_opt_gevent --report_multi_target || FAILED=true
+tools/run_tests/run_tests.py -l python --compiler python3.6 -c opt -t -x run_tests/python_linux_opt_native/sponge_log.xml --report_suite_name python_linux_opt_native --report_multi_target || FAILED=true
 
 if [ "$FAILED" != "" ]
 then