How can I reset my spawnpoint and spawn back at worldspawn?

The /spawnpoint command does not have a reset functionality built in, but there are other ways of doing this.

The simplest fix is to lie in a bed to set your spawnpoint to the bed, then destroy it and kill yourself. You will get the "Your home bed was missing or obstructed" message, and your personal spawnpoint will be completely unset.

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More technical explanation, tested with Minecraft 1.8.4 and NBTExplorer:

Using /spawnpoint will set 4 different NBT tags for the player: SpawnX, SpawnY, SpawnZ stores the coordinates, and SpawnForced is set to 1, which means the game will not look for a bed in the vicinity of your spawnpoint.

Lying in a bed removes the SpawnForced tag, and sets the coordinates. Next time you die, the game will look for a bed. If it is obstructed or destroyed, this will fail, at which point the game also removes the other three tags from your player.dat (or level.dat/player for singleplayer).