all: make use of sync.Map.Clear

all: make use of sync.Map.Clear

Since CL 515015 added sync.Map.Clear method, we can use it to make
the code simpler and clearer.

Change-Id: I29edc969431b4fd95cd5fd864953a71ca1538dd3
Reviewed-on: https://go-review.googlesource.com/c/go/+/582015
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Joedian Reid <joedian@google.com>
Reviewed-by: Damien Neil <dneil@google.com>