Remove all workarounds for Go < 1.11

internal/test/helper.go

@@ -1,5 +1,3 @@
-// +build go1.9
-
 package test
 
 import "testing"

internal/test/helper_go18.go

@@ -1,19 +0,0 @@
-// +build !go1.9
-
-package test
-
-import "testing"
-
-// Helperer marks the current function as a test helper.
-type Helperer interface {
-	Helper()
-}
-
-type fakeHelper struct{}
-
-func (fakeHelper) Helper() {}
-
-// Helper returns a function that marks the current function as a helper function.
-func Helper(t testing.TB) Helperer {
-	return fakeHelper{}
-}