Fix bug where resetting startup duration causes startup sprite to disappear.

What is the point of this PR?

To fix the bug mentioned here, where updating the startup duration of a sprite causes the image to no longer display in the “custom sprites” pane of the editor.

How did you do that?

By removing the line of code that replaced the URL of the image with the startup duration itself when the startup duration changed… Yes, really.

Are you sure it works?

Yes. I made a new trial, added a custom spite, updated the duration, clicked outside, and saw my custom sprite stayed where it was.

Other notes?

No.