Eliminate port server env var, force its use always, consolidate code

CMakeLists.txt

Makefile

build.yaml

test/core/util/BUILD

test/core/util/port.c

test/core/util/port_posix.c

test/core/util/port_server_client.c

test/core/util/port_server_client.h

test/core/util/port_windows.c

tools/run_tests/generated/sources_and_headers.json

tools/run_tests/python_utils/start_port_server.py

tools/run_tests/run_microbenchmark.py

tools/run_tests/run_tests.py

tools/run_tests/start_port_server.py

vsprojects/vcxproj/grpc_test_util/grpc_test_util.vcxproj

...cts/vcxproj/grpc_test_util/grpc_test_util.vcxproj.filters

...j/grpc_test_util_unsecure/grpc_test_util_unsecure.vcxproj

...est_util_unsecure/grpc_test_util_unsecure.vcxproj.filters