net: ieee802154: Remove IEEE802154_2015 option usage

The IEEE802154_2015 option is misleading, as it does not introduce full
802.15.4-2015 standard compliance but only random bits, plus it's
defined at the radio driver level, which brings yet another confusion.
Because of that, the option will be deprecated, and respective parts of
code that made use of it converted to use more specific configurations:

* nRF driver will now use CONFIG_NRF_802154_ENCRYPTION to specify
  whether to compile in TX security
* net_pkt will only add extra 802.15.4 control block fields if
  OpenThread is used, as they were solely used by this L2
* OpenThread radio layer will now use the OpenThread version to
  determine whether to compile in TX security.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>