How can I remove old tile mortar from a concrete floor?
Thinset mortar is usually fairly soft. For that small area you can probably knock down the lumps with a belt sander and coarse grit belt. You don't need to entirely remove the old mortar if it's well bonded. Just get a fairly flat top. Vacuum thoroughly before proceeding to remove all dust and debris.

Since you're presumably going to use a 1/4" notch trowel to set your tile, the mortar will accommodate the remaining roughness. It'll be a bit more difficult to apply the mortar than on fresh concrete, but no big deal.