net/http: remove unused doneChan
The https://golang.org/cl/43230 removed use of `getDoneChan`.
Change-Id: I33390c0e3aea6d98367363773ebe39d9c1f64ae9
GitHub-Last-Rev:
fe1e4154eaad4e485acfe4272f27d63367144e47
GitHub-Pull-Request: golang/go#53172
Reviewed-on: https://go-review.googlesource.com/c/go/+/409538
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: hopehook <hopehook@qq.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>