If the volumes array has multiple entries (which it probably does sometimes, otherwise there would be little reason to make it an array), then you could either add enough remove_field entries to cover all of them, or you could use a ruby filter to iterate over the array
ruby {
code => '
volumesArray = event.get("volumes")
if volumesArray
newVolumesArray = []
volumesArray.each { |x|
x.delete("lun-mapping-list")
newVolumesArray << x
}
event.set("volumes", newVolumesArray)
end
'
}