net: Deprecated and unused flags cleanup.

In net_pkt structure couple of flags are no longer used.
Flag pkt_queued can be removed permanently togeter with setter
and getter functions.
Second flag sent_or_eof has been renamed because it is still
used partially only for indicating EOF. Additionally unused setter
and getter for 'sent_' part of this flag are removed.

Signed-off-by: Marcin Gasiorek <marcin.gasiorek@nordicsemi.no>