doc: remove defunct playground.html

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5846071

doc/playground.html

-<!--{
-	"Title": "About the Go Playground"
-}-->
-
-<div class="left-column">
-<p>
-The Go Playground is a web service that runs on 
-<a href="http://golang.org/">golang.org</a>'s servers.
-The service receives a Go program, compiles, links, and runs the program inside 
-a sandbox, then returns the output.
-</p>
-
-<p>
-There are limitations to the programs that can be run in the Playground.
-They must be single-threaded (but they may use many goroutines).
-There are also limits on execution time, and CPU and memory usage.
-The Playground can access only a subset of the standard library
-(notably absent are network and file system access).
-Therefore, the only communication a Playground program has to the outside world
-is via standard output.
-</div>
-
-<div class="right-column">
-<script src="http://www.google.com/jsapi" type="text/javascript"></script>
-<div id="playground" class="small"></div>
-<script src="/doc/play/playground.js"></script>
-</div>
-
-<div class="end-columns"></div>