I will be tiling a powder room with 6 x 6 ceramic tiles. I have removed the 12x12 self-stick tiles that were on the floor and found that under these were the old 8x8 tiles that I think contained asbestoes. Thes tile are firmly attached to the concrete below. Should these be removed or can I leave them in place? If left in place would I use a mastic or thinset to install the new tile?