In this announcement post, it was announced that the API previously used to remove an Experience’s icon (POST https://www.roblox.com/places/icons/remove-icon) is being deprecated. The announcement post does not include a description for which API will replace it.
To remove an icon for an experience, you can call the Update Asset endpoint on the assets upload API. The path is like this: PATCH /v1/assets/{assetId}. Here is a link to the documentation for the API.
To remove the icon for an experience, pass in the placeId of its root place as the assetId. You can also pass the placeId of non-root places to remove the icon for a non-root place.
Remember to include the icon updateMask. An example request would look like this: