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

crypto/bls12381/g2.go

@@ -287,9 +287,8 @@ func (g *G2) Add(r, p1, p2 *PointG2) *PointG2 {
 	if t[1].equal(t[3]) {
 		if t[0].equal(t[2]) {
 			return g.Double(r, p1)
-		} else {
-			return r.Zero()
 		}
+		return r.Zero()
 	}
 	g.f.sub(t[1], t[1], t[3])
 	g.f.double(t[4], t[1])