StorageArea.remove()

let removingItem = browser.storage.<storageType>.remove(
  keys             // string, or array of strings
)

<storageType> will be one of the writable storage types — storage.sync or storage.local.

keys

A string, or array of strings, representing the key(s) of the item(s) to be removed.

A Promise that will be fulfilled with no arguments if the operation succeeded. If the operation failed, the promise will be rejected with an error message.

Remove a single item:

function onRemoved() {
  console.log("OK");
}

function onError(e) {
  console.log(e);
}

let removeKitten = browser.storage.sync.remove("kitten");
removeKitten.then(onRemoved, onError);