Remove dependency on emsdk

[jsturtevant]

Once dotnet/runtimelab#2592 is released we in a package we can remove emsdk.

Will need to:

• remove the download
• update wasi-sdk
• compile cabi_realloc.c with $WASI_SDK_PATH/bin/clang instead of emcc.bat
• update docs becuase it will work on Linux not just windows!