remove the deprecated `CloseNotify` function

[iawia002]

Is this a BUG REPORT or FEATURE REQUEST?:

Uncomment only one, leave it on its own line:

/kind bug /kind feature

What happened:

https://github.com/caicloud/nirvana/blob/854a4623c350ca2e71741cc93142f13e2b6afe8a/service/context.go#L208-L211

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

[kdada]

Just do it.