Is it possible to undo the georeferencing of a file-based raster?

Answer

No, it is not possible to undo the georeferencing of a file-based raster. If the raster is transformed permanently using the Rectify command (via the Georeferencing toolbar) or the Warp tool, the georeferencing operation cannot be undone.

However, if the raster is transformed using the Update Georeferencing command (via the Georeferencing toolbar), the georeferencing operation can be undone by deleting the transformation information stored in the auxiliary file.

The Update Georeferencing command stores the geodata transformation in an internal auxiliary file of the raster dataset. Georeferencing transformations of file-based raster datasets, such as a .tif files, are generally stored in an external auxiliary XML file. For more information on auxiliary files and where they can be found, refer to ArcGIS Help: Auxiliary files.

Note:
If the raster dataset is a raw image, such as a BMP file and the affine transformation method is used, the transformation information is stored in a world file. For more information, refer to ArcGIS Help: World files for raster datasets. 

Additionally, if working with CAD data, refer to ArcGIS Help: World files for CAD datasets for more information on how a world file is used to store georeferencing information.

To undo the georeferencing where the raster is transformed using the Update Georeferencing command:

1. Locate and open the auxiliary XML file of the raster.
2. Search for the <Metadata> XML tag containing the <GeodataXForm> tag. For example:

<Metadata domain="xml:ESRI" format="xml">
    <GeodataXform xmlns:typens="http://www.esri.com/schemas/ArcGIS/10.4"
    xmlns:xs="http://www.w3.org/2001/XMLSchema"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="typens:PolynomialXform">
    ......
    </GeodataXform>
</Metadata>

3. Delete the <Metadata> XML tag section containing the <GeodataXForm> tag > Save.
4. Add the raster to ArcMap.