[release-branch.go1.13] net/http: remove http2 connections when no longer cached

[release-branch.go1.13] net/http: remove http2 connections when no longer cached

When the http2 transport returns a NoCachedConnError, the connection
must be removed from the idle list as well as the connections per host.

Fixes #34498

Change-Id: I7875c9c95e694a37a339bb04385243b49f9b20d3
Reviewed-on: https://go-review.googlesource.com/c/go/+/196665
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/197377
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>