Fix toString() method for Animation expression

[elenatorro]

Context

When creating an AnimationGeneral class, on the _init function we're adding the progress to the animation childrenNames. Therefore, when using the .toString() method inherited from the Base expression class, it includes the progress because this method goes through all the expression children names.

Example

const s = carto.expressions;
const animation = s.animation(s.prop('mydate'), 30, s.fade(1, 1));

animation.toString()

// Current result:
"animation($mydate, 30, fade(1, 1), 0.12345)"

// Expected result
"animation($mydate, 30, fade(1, 1))"