Revert "net: tcp: Queue FIN instead of sending it immediately"

This reverts commit 817245c5.

In certain cases the peer seems to discard the FIN packet we are
sending, which means that the TCP stream is not closed properly.
This needs more work so revert this for time being.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>