Remove vs2015 test (grpc#29824)

Author: veblush

tools/run_tests/run_tests_matrix.py

@@ -329,7 +329,7 @@ def _create_portability_test_jobs(extra_args=[],
                                 configs=['dbg'],
                                 platforms=['windows'],
                                 arch='default',
-                                compiler='cmake_vs2015',
+                                compiler='cmake_vs2017',
                                 labels=['portability', 'corelang'],
                                 extra_args=extra_args,
                                 inner_jobs=inner_jobs)