net: openthread: radio: Removed retranssmisions from radio caps.

Zephyr platform does not support MAC retransmissions on its own,
so OT_RADIO_CAPS_TRANSMIT_RETRIES capability was removed.
It should not be enabled basing on IEEE802154_HW_CSMA support,
as these are quite seperate features. Current implementation
assumes that platform performs retransmissions on its own,
what is not provided and leads to lack of MAC retransmissions.

Signed-off-by: Kamil Kasperczyk <kamil.kasperczyk@nordicsemi.no>