[cleanup] Removed 'Is Service Item' checkbox and filters

'Is Service Item' checkbox only worked for filtering Items in Maintenance Schedule and Maintenance Visit, and validating that Items in a Maintenance type Order were of type 'Service'. However, it doesn't fit an actual use case where any Sales Item could be given for Maintenance, making the checkbox an unnecessary addition.
diff --git a/erpnext/stock/get_item_details.py b/erpnext/stock/get_item_details.py
index 1c0acce..5548350 100644
--- a/erpnext/stock/get_item_details.py
+++ b/erpnext/stock/get_item_details.py
@@ -111,11 +111,7 @@
 
 	if args.transaction_type=="selling":
 		# validate if sales item or service item
-		if args.get("order_type") == "Maintenance":
-			if item.is_service_item != 1:
-				throw(_("Item {0} must be a Service Item.").format(item.name))
-
-		elif item.is_sales_item != 1:
+		if item.is_sales_item != 1:
 			throw(_("Item {0} must be a Sales Item").format(item.name))
 
 		if cint(item.has_variants):