remove the deprecated `CloseNotify` function

Is this a BUG REPORT or FEATURE REQUEST?:

Uncomment only one, leave it on its own line:

/kind bug
/kind feature

What happened:

Lines 208 to 211 in 854a462

    
           // CloseNotify is a disguise of http.response.CloseNotify(). 
        
           func (c *response) CloseNotify() <-chan bool { 
        
           	return c.writer.(http.CloseNotifier).CloseNotify() 
        
           }

the original CloseNotifier interface was marked as Deprecated since Go 1.11, the user should use http.Request.Context.Done() as instead. Should we remove this too?

PS: this will trigger a lint problem (#287)

What you expected to happen:

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:

/cc @kdada

Just do it.

	// CloseNotify is a disguise of http.response.CloseNotify().
	func (c *response) CloseNotify() <-chan bool {
	return c.writer.(http.CloseNotifier).CloseNotify()
	}