all: simplify nested complexity and if blocks ending with a return statement (#21854)

Changes:

    Simplify nested complexity
    If an if blocks ends with a return statement then remove the else nesting.

Most of the changes has also been reported in golint https://goreportcard.com/report/github.com/ethereum/go-ethereum#golint

p2p/nat/nat.go

@@ -219,9 +219,8 @@ func (n *autodisc) String() string {
 	defer n.mu.Unlock()
 	if n.found == nil {
 		return n.what
-	} else {
-		return n.found.String()
 	}
+	return n.found.String()
 }
 
 // wait blocks until auto-discovery has been performed.