net: tcp: Remove net_tcp_unref()

There is a pending TODO regarding unifying net_tcp_unref() and
net_tcp_put(). Given that net_tcp_unref() is no longer used by the upper
layer (it should only use get/put APIs), the function can be removed
from external TCP API, as referencing/dereferencing is now only used
internally by the TCP stack.

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