extensions-app: Stop setting description tooltip

We no longer ellipsize the description label, so the tooltip
no longer has a purpose.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3051>

subprojects/extensions-app/js/main.js

@@ -428,7 +428,6 @@ var ExtensionRow = GObject.registerClass({
 
         const desc = this._extension.metadata.description.split('\n')[0];
         this._descriptionLabel.label = desc;
-        this._descriptionLabel.tooltip_text = desc;
 
         this.connect('destroy', this._onDestroy.bind(this));