sysutils/nut-devel: Fix plist

Upstream doesn't appear to distribute man pages, in the traditional
sense, anymore. Looking at their git logs, this appears to be
intentional. They say this is to make it easier for distributions,
i.e. us, to package man pages. IMO this is not the case for FreeBSD.
In cbdbd520abc6 I simply copied the man pages to $LOCALBASE/share/doc.
This of course will need to change.

In the mean time the MAN option has been removed and man pages will
remain in share/doc until I can develop a script (in Makefile) to
read the man page volume number in the first line of each .txt file
in order to install the man pages into their correct places in
$LOCALBASE.

This commit temporarily works around the failing build.