Update and clean up AssemblyScript example (#4233)

* Update and clean up AssemblyScript example

* Remove wasm build files

* Remove wasm source files

docs/assemblyscript-hello-world/asconfig.json

@@ -0,0 +1,9 @@
+{
+    "targets": {
+        "helloworld": {
+            "outFile": "build/wasi-hello-world.wasm",
+            "textFile": "build/wasi-hello-world.wat"
+        }
+    },
+    "options": {}
+}