How to remove dust from outlet holes?
It's generally not a safety hazard unless there's so much dust that it begins to obstruct the insertion of cord prongs. Use a strong vacuum with a non-metallic nozzle (mostly to avoid scratching the outlet covers), and press it tightly against the outlet face to create a seal.
If you find the the dust is sticky, use a soft bristle brush to agitate the dust, then vacuum.
If the outlets are more than about 20 years old they should be replaced anyway. Over time the electrical contacts wear and weaken, leading to insecure connections, sparking, and eventually failure.