Disable non-functional pnp4nagios icon next to services that doesn't provide perfdata

i.e. https://unstable.bitmask.net/cgi-bin/nagios3/extinfo.cgi?type=2&host=ant.demo.bitmask.i&service=Bigcouch_epmd_procs

(from redmine: created on 2015-01-13, relates #1539 (closed))