Resolve "CID 357292: Improper use of negative value in lib/isc/netmgr/tcp.c"
Remove a superfluous check of sock->fd against -1.
The check comes from the times when tcp_connect_direct() was performing isc__nm_socket() by its own, but currently at this point the isc__nm_socket() is already passed and sock->fd cannot be -1.
Closes #3574 (closed)