godoc: more cleanup

cmd/godoc/godoc.go is now merged into main.go, which is now
only 530 lines.

App Engine mode is still broken, but should be easy to fix up.
(just needs a global *godoc.Presentation created in init)

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/11498044
diff --git a/godoc/godoc.go b/godoc/godoc.go
index 093f007..ee55355 100644
--- a/godoc/godoc.go
+++ b/godoc/godoc.go
@@ -375,12 +375,12 @@
 			out = ""
 		}
 
-		if ExampleHTML == nil {
+		if p.ExampleHTML == nil {
 			out = ""
 			return ""
 		}
 
-		err := ExampleHTML.Execute(&buf, struct {
+		err := p.ExampleHTML.Execute(&buf, struct {
 			Name, Doc, Code, Play, Output string
 		}{eg.Name, eg.Doc, code, play, out})
 		if err != nil {