net/http/pprof: remove comments pointing to gperftools

The version of pprof in gperftools has been deprecated.
No need to have a pointer to that version since go tool pprof
is included with the Go distro.

Change-Id: I6d769a68f64280f5db89ff6fbc67bfea9c8f1526
Reviewed-on: https://go-review.googlesource.com/24509Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/pprof/pprof.go

@@ -4,8 +4,6 @@
 
 // Package pprof serves via its HTTP server runtime profiling data
 // in the format expected by the pprof visualization tool.
-// For more information about pprof, see
-// http://code.google.com/p/google-perftools/.
 //
 // The package is typically only imported for the side effect of
 // registering its HTTP handlers.