I have had good success removing brick with an air chisle. Its a small handheld accessory that cost about $30 but you will need an air compressor. Basically a mini-jackhammer, it allows brick to be removed at the mortar joints or splits bricks fairly cleanly, and will clean up the thinset or mortar bedding. Don't know what you will find under the brick on your fireplace, so be prepared to do more than you planned.