log/slog: remove obsolete comment

The comment was correct when written but became obsolete
when the mu field changed to a pointer to a Mutex.
The code is clear in any case.

Change-Id: Ibef2c2c4df07050e73e92bd14fabccdd3b48b0da
Reviewed-on: https://go-review.googlesource.com/c/go/+/789400
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>

src/log/slog/handler.go

diff --git a/src/log/slog/handler.go b/src/log/slog/handler.go
index 26eb4b8..691a080 100644
--- a/src/log/slog/handler.go
+++ b/src/log/slog/handler.go
@@ -204,7 +204,6 @@
 }
 
 func (h *commonHandler) clone() *commonHandler {
-	// We can't use assignment because we can't copy the mutex.
 	return &commonHandler{
 		json:              h.json,
 		opts:              h.opts,