You have a Custom or Enterprise CUIX file, and you have custom button icons in it that the source BMP files have gone missing or you have Custom Button icons that you don't want to use, and want to remove them from the CUIX file.
This is because when you save a Custom CUIX (or even save over a generic one with custom icons), they are added into the CUIX file. If the icons go missing from a support directory, you can generally restore the buttons by changing the Theme from light to dark and vice versa to get the icons back until you close and re-open the software.
First thing to do is find the CUIX file that has the Icons in it.
1. Make a copy of that CUIX file somewhere else on your system (I like C:\Temp or C:\Autodesk)
2. Change the Extension of the file from CUIX to ZIP
b. NEVER work off the live file, always make a copy and work from the copy, in case you make a mistake
b. Remove the .BMP file located in the root of the file
c. Exit the Editor
d. Rename the ZIP to CUIX
e. Rename the original file to .old, copy the corrected one to that location.
f. Open Autodesk software and confirm the Custom icons no longer show up in the Editor
c. Copy them to the Icons folder (or other support folder)
d. Relaunch the Autodesk product and confirm all Icons on the toolbars/ribbon are now showing (no clouds)