wgengine/magicsock: remove mention of Start

The Start method was removed in 4c27e2fa22, but the comment on NewConn
still mentioned it doesn't do anything until this method is called.

Signed-off-by: Kris Brandow <kris.brandow@gmail.com>

wgengine/magicsock/magicsock.go

@@ -538,8 +538,6 @@ func newConn() *Conn {
 // NewConn creates a magic Conn listening on opts.Port.
 // As the set of possible endpoints for a Conn changes, the
 // callback opts.EndpointsFunc is called.
-//
-// It doesn't start doing anything until Start is called.
 func NewConn(opts Options) (*Conn, error) {
 	c := newConn()
 	c.port.Store(uint32(opts.Port))