Remove the first instance of `item`

Property definitions

	# Remove the first instance of `item`
	fun remove(item: E)
	do
		var index = items.index_of(item)
		remove_at(item, index)
	end

lib/gamnit/flat/flat_core.nit:1600,2--1605,4

	redef fun remove(item) do remove_at(item, item.context_index)

lib/gamnit/flat/flat_core.nit:1729,2--62