net: tcp: remove net_tcp_set_state() function

There are no users of net_tcp_set_state() left outside of
subsys/net/ip/tcp.c.

And the naming of this function is confusing -- it could easily
be mistaken for net_tcp_change_state() which contains additional
logic for certain tcp states.

Let's remove it entirely and fix the remaining uses to set
tcp->state directly.

Change-Id: I92855ad180e8682780fcff11e50af06adcbc177c
Signed-off-by: Michael Scott <michael.scott@linaro.org>