naemon/xrddefault.c: removing references to Nagios(tm)

removed some old references to Nagios(tm) from retention file.

Signed-off-by: Ricardo Maraschini <ricardo.maraschini@gmail.com>
Merge pull request #48 from ricardomaraschini/retention-strings

naemon/xrddefault.c: removing references to Nagios(tm)