context: replace newAfterFuncContext with a global declaration using blank identifier

newAfterFuncContext has never been used, the only reason I can imagine
for its existence is to guarantee that the implementation is correct.

It is a small cleanup and make code more idiomatic.

Change-Id: Ib62bd3f9f546aabe4ea5de06fdf3e6218b6d555c

src/context/afterfunc_test.go

@@ -20,9 +20,7 @@ type afterFuncContext struct {
 	err        error
 }
 
-func newAfterFuncContext() context.Context {
-	return &afterFuncContext{}
-}
+var _ context.Context = (*afterFuncContext)(nil)
 
 func (c *afterFuncContext) Deadline() (time.Time, bool) {
 	return time.Time{}, false