Remove __BSD_VISIBLE from spiped build

I added this in 2013 because it was needed in order to have MSG_NOSIGNAL
defined; having that improved performance by avoiding the need go mask
and unmask SIGPIPE repeately.

From src r248932 onwards (aka. 10.0-RELEASE and later) __BSD_VISIBLE is
no longer needed for this; since FreeBSD 9.x and earlier are no longer
supported, there is no point keeping the workaround.