I'd experiment with a hair dryer on the exposed flypaper stuff to see if that assists in removing it. If it comes off, then I'd proceed with a relatively thin putty knife positioned at a super shallow angle (so it doesn't want to penetrate into the drywall) between the drywall and tiles. Tap the top of the handle with a hammer to drive the putty knife forward to separate the tiles. Do protect yourself and the area from the breaking glass.
If that doesn't want to work, you could just remove the drywall (tile and all) and replace it. Cut around the perimeter of the tile and pull it out. If you can't make the "bottom cut" along the countertop because the tile is in the way, make the top and whatever side cuts you can make and pull the board forward...it will break small or large sections of drywall out at a time. Not exactly clean, but could be pretty effective. Drywall is cheap, so this might be an attractive option over trying to slowly remove and clean the tiles.