sysutils/nut: Remove redundant BUILD_DEPENDS

Remove unused asciidoc BUILD_DEPENDS from sysutils/nut as its man pages
are already preformatted.

Also, add a comment to the sysutils/nut-devel port Makefile informing
developers to remove the BUILD_DEPENDS when copying/merging the -devel
port to the main port when a new release of nut is committed.

PR: 263751
Reported by: Joerg Pulz <Joerg.Pulz@frm2.tum.de>