Delete test of premature optimization

I suspect this is worth some perf, but it's probably not enough for us
to care. More importantly, it breaks in Go 1.5, where a recent
optimization made sync.WaitGroup much smaller.

graceful/listener/shard_test.go

@@ -1,21 +0,0 @@
-// +build amd64
-
-package listener
-
-import (
-	"testing"
-	"unsafe"
-)
-
-// We pack shards together in an array, but we don't want them packed too
-// closely, since we want to give each shard a dedicated CPU cache line. This
-// test checks this property for x64 (which has a 64-byte cache line), which
-// probably covers the majority of deployments.
-//
-// As always, this is probably a premature optimization.
-func TestShardSize(t *testing.T) {
-	s := unsafe.Sizeof(shard{})
-	if s < 64 {
-		t.Errorf("sizeof(shard) = %d; expected >64", s)
-	}
-}