a few more cleanup bits

Author: nshahan

frontend_server_client/example/web_client.dart

@@ -27,7 +27,6 @@ void main(List<String> args) async {
       '--multi-root-scheme=org-dartlang-sdk',
       '--modules=amd',
       '--module-name=dart_sdk',
-      '--sound-null-safety',
       '-o',
       dartSdkJs,
       p.url.join(sdkDir, sdkKernelPath),

frontend_server_common/lib/src/frontend_server_client.dart

@@ -394,7 +394,6 @@ class ResidentCompiler {
       ],
       if (useDebuggerModuleNames) '--debugger-module-names',
       '--experimental-emit-debug-metadata',
-      '--sound-null-safety',
       for (final experiment in compilerOptions.experiments)
         '--enable-experiment=$experiment',
       if (compilerOptions.canaryFeatures) '--dartdevc-canary',