net: tcp: Remove pointless recv callback calls

Calling the registered receive callback when releasing TCP context
doesn't make sense, as at that point the application should've already
closed the associated socket (that's one of the conditions for the
context to be released). Therefore, remove the pointless receive
callback call, while keeping the loop to unref any leftover data packets
(although again, I don' think there should be any packets left at that
point, as they're all consumed in tcp_in()).

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