Simplify function lists in RemoveUnusedModuleElements (NFC) (#2147)
Simplify function lists in RemoveUnusedModuleElements (NFC) (#2147)

It looks there's no need to maintain `functions` and `functionImports`
separately.
WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen,WebAssembly/binaryen