Yes, you can remove it or if you wish tile over it.
Most likely you have an older home with 1" x 1" tile installed over a 2" thick mud bed installed over wire lathe and roofing felt. If the mud bed is fractured up through the tile then you should not tile over it. Install an anti-fracture membrane similiar to Schlüter-DITRA. If it is not fractured then removing the tile should not be too difficult because most of these old installations did not use thinset mortar as an adhesive. The tiles should pop loose with little effort with a chisel and hammer once you have established a good "starting point". Under the toilet may be the best place to begin. Removing the tile will offer you a premium bonding surface as well as improved floor height.
To install tile directly over the tile you will first need to prepare the existing tile surface by scrubbing the tile and grout joints with an acid based tile cleaner. Rinse well with clean water and allow to dry. Any loose tiles should be removed. Even sanding the surface of the tiles will greatly improve the mechanical bond of the thinset mortar to the existing tile surface. Install tile using a latex modified thinset mortar with a 1/4" - 3/8" thick thinset mortar bed using a square notch trowel.