Go to your gallery and select a specific layout, then look at the bottom under Additional Info. You'll see several icons. On the bottom row at the left there's one that looks like a stamp. Click this to get to the edit screen. Then scroll all the way to the bottom of this screen. You'll see the option to delete. Check the box then click Submit Changes and your layout will be gone from the gallery. But as the warning notes, once it's gone it's gone.

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