Fix stupid condition when removing items

[Enrique Alfonso Casanovas Pedre]

The if conditions are wrong. children is never empty, since it's the array of children when the node is a Branch, it's never empty and I ask if it's empty.
I have to ask if the newBitmap is empty instead.
The same happens for the elif condition. It's length is never 1. I must ask whether the newBitmap contains only one bit on.

| Branch (bitmap, children) ->
    let bitIndex = childBitIndex prefix

    if containsChild bitIndex bitmap then
        let childArrayIndex = childArrayIndex bitIndex bitmap

        match remove eqComparer targetKey targetHash (nextLayerPrefix prefix) (Array.item childArrayIndex children)
            with
        | NotFound -> NotFound
        | Removed child -> Removed(Branch(bitmap, Array.put child childArrayIndex children))
        | NothingLeft ->
            let newBitmap = Bitmap.clearBit bitIndex bitmap

            if Array.isEmpty children then
                NothingLeft
            elif Array.length children = 1 then
                Removed(Array.head children)
            else
                Removed(Branch(newBitmap, Array.remove childArrayIndex children))
    else
        NotFound