[release-branch.go1.15] sync: delete dirty keys inside Map.LoadAndDelete

Updates #40999
Fixes #41011

Change-Id: Ie32427e5cb5ed512b976b554850f50be156ce9f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/250197


Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit 94953d3e)
Reviewed-on: https://go-review.googlesource.com/c/go/+/250297


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>