sync: remove the redundant logic on sync.(*Pool).Put

When the procUnpin is placed after shared.pushHead, there is
no need for x as a flag to indicate the previous process.

This CL can make the logic clear, and at the same time reduce
a redundant judgment.

Change-Id: I34ec9ba4cb5b5dbdf13a8f158b90481fed248cf5
Reviewed-on: https://go-review.googlesource.com/c/go/+/360059
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/sync/pool.go

@@ -102,9 +102,7 @@ func (p *Pool) Put(x any) {
 	l, _ := p.pin()
 	if l.private == nil {
 		l.private = x
-		x = nil
-	}
-	if x != nil {
+	} else {
 		l.shared.pushHead(x)
 	}
 	runtime_procUnpin()