ganglia: remove unused reference to neon

This follows up & corrects commit
db1c635b8ed3b5329443c61a442f3dfb9470dfd9.

Two issues:
a) Contrary to what the comment & commit message of the above says,
check_ganglia does not need a local gmetad. Our default check_ganglia
config connects to uranium, the main Ganglia aggregator. Having a local
gmetad just creates unnecessary I/O & CPU load.

b) Even if that was the case, ganglia::collector is never included to
neon, the icinga box. The config stanza is right now unused and never
updated, so remove it.

Change-Id: I79b228eea7d00f0b0ab342f57ac73f65084fcba0