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

les/serverpool.go

@@ -112,9 +112,8 @@ var (
 				}
 				enc, err := rlp.EncodeToBytes(&ne)
 				return enc, err
-			} else {
-				return nil, errors.New("invalid field type")
 			}
+			return nil, errors.New("invalid field type")
 		},
 		func(enc []byte) (interface{}, error) {
 			var ne nodeHistoryEnc