Revert "[pigeon]enable treat warning as errors for swift code in unit test" (flutter#3898)

Reverts flutter#3889

The tree is red on a warning in the test code, most likely new in the Xcode version that the tree was just updated to. Reverting so we can re-land this with a fix for the new warning.

Author: stuartmorgan-g

packages/pigeon/platform_tests/test_plugin/example/ios/Runner.xcodeproj/project.pbxproj

@@ -542,7 +542,6 @@
 				PRODUCT_NAME = "$(TARGET_NAME)";
 				SWIFT_ACTIVE_COMPILATION_CONDITIONS = DEBUG;
 				SWIFT_OPTIMIZATION_LEVEL = "-Onone";
-				SWIFT_TREAT_WARNINGS_AS_ERRORS = YES;
 				SWIFT_VERSION = 5.0;
 				TEST_HOST = "$(BUILT_PRODUCTS_DIR)/Runner.app/$(BUNDLE_EXECUTABLE_FOLDER_PATH)/Runner";
 			};
@@ -558,7 +557,6 @@
 				MARKETING_VERSION = 1.0;
 				PRODUCT_BUNDLE_IDENTIFIER = dev.flutter.plugins.RunnerTests;
 				PRODUCT_NAME = "$(TARGET_NAME)";
-				SWIFT_TREAT_WARNINGS_AS_ERRORS = YES;
 				SWIFT_VERSION = 5.0;
 				TEST_HOST = "$(BUILT_PRODUCTS_DIR)/Runner.app/$(BUNDLE_EXECUTABLE_FOLDER_PATH)/Runner";
 			};
@@ -574,7 +572,6 @@
 				MARKETING_VERSION = 1.0;
 				PRODUCT_BUNDLE_IDENTIFIER = dev.flutter.plugins.RunnerTests;
 				PRODUCT_NAME = "$(TARGET_NAME)";
-				SWIFT_TREAT_WARNINGS_AS_ERRORS = YES;
 				SWIFT_VERSION = 5.0;
 				TEST_HOST = "$(BUILT_PRODUCTS_DIR)/Runner.app/$(BUNDLE_EXECUTABLE_FOLDER_PATH)/Runner";
 			};