docs/Wasm: remove incompatible --libcxx flags for build-script (#86017)

Freshly built libc++ introduces issues when building `swift-build`:

```
Undefined symbols for architecture arm64:
  "std::__1::__hash_memory(void const*, unsigned long)", referenced from:
      (anonymous namespace)::CAPIBuildDB::buildUpKeyCache(std::__1::vector<llbuild::core::KeyType, std::__1::allocator<llbuild::core::KeyType>>&) in BuildDB-C-API.cpp.o
      _llb_build_key_make in BuildKey-C-API.cpp.o
      _llb_build_key_make_command in BuildKey-C-API.cpp.o
      _llb_build_key_make_custom_task in BuildKey-C-API.cpp.o
      _llb_build_key_make_custom_task_with_data in BuildKey-C-API.cpp.o
      _llb_build_key_make_directory_contents in BuildKey-C-API.cpp.o
      _llb_build_key_make_filtered_directory_contents in BuildKey-C-API.cpp.o
      ...
ld: symbol(s) not found for architecture arm64
```

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Author: MaxDesiatov

docs/WebAssembly.md

@@ -73,7 +73,7 @@ for WebAssembly is built and tested using the following command (exclude `--scca
 
 ```bash
 ./utils/build-script --sccache --build-wasm-stdlib --wasmkit --install-llvm --install-swift --swiftpm --install-swiftpm \
-  --libcxx --install-libcxx --llbuild --install-llbuild --swift-testing --install-swift-testing \
+  --llbuild --install-llbuild --swift-testing --install-swift-testing \
   --swift-testing-macros --install-swift-testing-macros --build-embedded-stdlib --build-embedded-stdlib-cross-compiling \
   '--llvm-install-components=llvm-ar;llvm-nm;llvm-ranlib;llvm-cov;llvm-profdata;llvm-objdump;llvm-objcopy;llvm-symbolizer;IndexStore;clang;clang-resource-headers;builtins;runtimes;clangd;libclang;dsymutil;LTO;clang-features-file;lld'
 ```