Grab shipment line items via inventory units

The previous implementations doesn't make sense anymore on the current
split shipments scenario. Can't see a reason why tracking inventory
levels config should influence whether or not the line item should be
returned

core/app/models/spree/shipment.rb

@@ -161,11 +161,7 @@ def manifest
     end
 
     def line_items
-      if order.complete? and Spree::Config.track_inventory_levels
-        order.line_items.select { |li| !li.should_track_inventory? || inventory_units.pluck(:variant_id).include?(li.variant_id) }
-      else
-        order.line_items
-      end
+      inventory_units.includes(:line_item).map(&:line_item).uniq
     end
 
     def finalize!

core/spec/models/spree/shipment_spec.rb

@@ -203,38 +203,6 @@
     end
   end
 
-  context "when track_inventory is false" do
-    before { Spree::Config.set track_inventory_levels: false }
-    after { Spree::Config.set track_inventory_levels: true }
-
-    it "should not use the line items from order when track_inventory_levels is false" do
-      line_items = [mock_model(Spree::LineItem)]
-      order.stub complete?: true
-      order.stub line_items: line_items
-      shipment.line_items.should == line_items
-    end
-  end
-
-  context "when variant inventory tracking is false" do
-    it "should include line items without inventory if variant inventory tracking is off" do
-      line_items = [mock_model(Spree::LineItem)]
-      line_items.each { |li| li.stub should_track_inventory?: false }
-      order.stub complete?: true
-      order.stub line_items: line_items
-      shipment.line_items.should == line_items
-    end
-
-    it "should not include line items without inventory if variant inventory tracking is on" do
-      line_items = [mock_model(Spree::LineItem)]
-      line_items.each { |li| li.stub should_track_inventory?: true }
-      order.stub complete?: true
-      order.stub line_items: line_items
-      shipment.line_items.should == []
-    end
-  end
-
-
-
   context "when order is completed" do
     after { Spree::Config.set track_inventory_levels: true }