http2: remove unused variable

Leftover from https://golang.org/cl/18932

Change-Id: I41c1adfdebd7dff0f75c2ba2b669f13b163984a2
Reviewed-on: https://go-review.googlesource.com/18936
Reviewed-by: Ian Lance Taylor <iant@golang.org>

http2/server.go

@@ -484,8 +484,6 @@ func (sc *serverConn) logf(format string, args ...interface{}) {
 	}
 }
 
-var uintptrType = reflect.TypeOf(uintptr(0))
-
 // errno returns v's underlying uintptr, else 0.
 //
 // TODO: remove this helper function once http2 can use build