Bluetooth: samples: hci_uart: Remove unsupported configuration

Remove BT_CTLR_DTM_HCI=y config which is not supported for this board.
This produced the following warning:

warning: BT_CTLR_DTM_HCI was assigned the value
'y' but got the value 'n'. Check these unsatisfied dependencies:
BT_CTLR_DTM_HCI_SUPPORT (=n).

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>