Bluetooth: hci_uart: Remove unused h4->rx_skb field

The h4->rx_skb is not used anymore and with that just remove it. Seems
this was a leftover and even the kfree_skb call freeing it is rather
pointless since it got never allocated.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>