mptcp: cleanup writer wake-up

After commit 5cf92bba ("mptcp: re-enable sndbuf autotune"), the
MPTCP_NOSPACE bit is redundant: it is always set and cleared together with
SOCK_NOSPACE.

Let's drop the first and always relay on the latter, dropping a bunch
of useless code.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>