net/http2: remove awaitGracefulShutdown

It was added by https://golang.org/cl/43455 and
its usage was removed by https://golang.org/cl/43230

Updates golang/go#20302

Change-Id: I5072c3d9cbf9a33d2ac613bc5a3c059dc54e9d29
GitHub-Last-Rev: 68a32fb702168992427174c41c5d4638f4e567ad
GitHub-Pull-Request: golang/net#184
Reviewed-on: https://go-review.googlesource.com/c/net/+/509117
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
diff --git a/http2/server.go b/http2/server.go
index 033b6e6..6d5e008 100644
--- a/http2/server.go
+++ b/http2/server.go
@@ -1012,14 +1012,6 @@
 	}
 }
 
-func (sc *serverConn) awaitGracefulShutdown(sharedCh <-chan struct{}, privateCh chan struct{}) {
-	select {
-	case <-sc.doneServing:
-	case <-sharedCh:
-		close(privateCh)
-	}
-}
-
 type serverMessage int
 
 // Message values sent to serveMsgCh.