function removeOldTextNode(el) {
    if (el && el.__textSvgEl) {
        // textSvgEl may has no parentNode if el has been removed temporary.
        if (el.__textSvgEl.parentNode) {
            el.__textSvgEl.parentNode.removeChild(el.__textSvgEl);
        }
        el.__textSvgEl = null;
        el.__tspanList = [];
        el.__text = null;
    }
}
Get the <defs> tag for svgRoot; optionally creates one if not exists.

@param {boolean} isForceCreating if need to create when not exists
@return {SVGDefsElement} SVG <defs> element, null if it doesn't
exist and isForceCreating is false