How to delete ender dragon in unloaded chunks - overworld
Solution 1:
If the chunk is unloaded, the dragon can't move. Only mobs in loaded chunks can move.
Solution 2:
If your scared, make a repeating command block at spawn that does/kill @e[type=Minecraft:ender_dragon]. Make sure it's always active and repeating. If it's not at spawn, make sure to force load the chunk using /forceload add ~ ~. This will instantly kill the dragon once it's loaded in.