If you want to remove all the materials applied then in the Modify Panel select the Materials Tab and then delete everything that you don't wish to have by selecting each item in the list.

Another option is to select every material at once and reduce the strength of the Base Color to 0(Zero) - Select the first item in the list, hold the Shift Kae and then click on the last item.

Doing this will keep the Bump and other channels while removing the material colour. If you wish you can also select each channel and reduce the strength to zero.
Gerry
System: Win 10 Pro (21H2), Asus X99-E WS, CPU i7-5930K -3,5 GHz, 32 GB DDR4 2666-16 RAM, NVidia GTX 1080 Ti GPU - 11 GB VRAM(Driver Studio-536.99), Samsung 850 Pro 512 GB SSD, 6 TB HD storage.