Revert "MAINTAINERS: mptcp@ mailing list is moderated"

This reverts commit 74759e16.

mptcp@lists.01.org accepts messages from non-subscribers. There was an
invisible and unexpected server-wide rule limiting the number of
recipients for subscribers and non-subscribers alike, and that has now
been turned off for this list.

Cc: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>