Remove non-working example. Postman-echo does not return CORS headers. After a quick search I did not find a test echo server which returned CORS headers. (#1002)

I have rolled an example header into the other WASM example

examples/wasm_github_fetch/README.md

@@ -1,6 +1,10 @@
 ## Example usage of Reqwest from WASM
 
-You can build the example locally with:
+Install wasm-pack with
+
+    npm install
+
+Then you can build the example locally with:
 
 
     npm run serve