fix(asset): recompute reference doc reqd on asset type change

The asset_type change handler no longer called toggle_reference_doc, so
switching the asset type never cleared the mandatory flags on
purchase_receipt/purchase_invoice, throwing a mandatory error on the
first save.

erpnext/assets/doctype/asset/asset.js

@@ -571,6 +571,7 @@ frappe.ui.form.on("Asset", {
 				frm.set_df_property("net_purchase_amount", "read_only", 0);
 			}
 		}
+		frm.trigger("toggle_reference_doc");
 	},
 
 	create_asset_maintenance: function (frm) {