[release-branch.go1.13] net/http: only decrement connection count if we removed a connection

The connection count must only be decremented if the persistent
connection was also removed.

Fixes #36583

Change-Id: I5070717d5d9effec78016005fa4910593500c8cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/202087


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/+/215177


Run-TryBot: Alexander Rakoczy <alex@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>