http2: remove PingTimeout from TestTransportPingWhenReading

Use the default PingTimeout since it has no bearing on the test. A small
value can cause a failure on slower machines. Rely on the deadline to
determine a sufficient amount of time to complete.

Fixes golang/go#48668

Change-Id: I9389777fa00ed5193f1fc7ae04d2e2134114c675
Reviewed-on: https://go-review.googlesource.com/c/net/+/352970
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Bryan C. Mills <bcmills@google.com>
Trust: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
diff --git a/http2/transport_test.go b/http2/transport_test.go
index 3eb133f..0c35ca1 100644
--- a/http2/transport_test.go
+++ b/http2/transport_test.go
@@ -3437,7 +3437,6 @@
 func testTransportPingWhenReading(t *testing.T, readIdleTimeout, deadline time.Duration, expectedPingCount int) {
 	var pingCount int
 	ct := newClientTester(t)
-	ct.tr.PingTimeout = 10 * time.Millisecond
 	ct.tr.ReadIdleTimeout = readIdleTimeout
 
 	ctx, cancel := context.WithTimeout(context.Background(), deadline)