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

rlp/encode_test.go

@@ -39,9 +39,8 @@ func (e *testEncoder) EncodeRLP(w io.Writer) error {
 	}
 	if e.err != nil {
 		return e.err
-	} else {
-		w.Write([]byte{0, 1, 0, 1, 0, 1, 0, 1, 0, 1})
 	}
+	w.Write([]byte{0, 1, 0, 1, 0, 1, 0, 1, 0, 1})
 	return nil
 }