Revert "Force use of fallback chanpool on App Engine"

This has been fixed in v1.9.17 of App Engine:
https://code.google.com/p/googleappengine/issues/detail?id=11346

This reverts commit 9556e1736d.

Fixes #67.

web/chanpool.go

@@ -1,4 +1,4 @@
-// +build !go1.3 appengine
+// +build !go1.3
 
 package web
 

web/cpool.go

@@ -1,4 +1,4 @@
-// +build go1.3,!appengine
+// +build go1.3
 
 package web