Remove obsolete order.xsd file

The order.xsd is no longer used by any tests and probably has not been
since commit 89a7e752ef, which removed support for Castor.

See gh-22250

spring-oxm/src/test/resources/org/springframework/oxm/order.xsd

@@ -1,18 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<schema xmlns="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified"
-        targetNamespace="https://samples.springframework.org/order"
-        xmlns:tns="https://samples.springframework.org/order">
-    <element name="order">
-        <complexType>
-            <sequence>
-                <element name="order-item" type="tns:orderItemType"
-                         maxOccurs="unbounded">
-                </element>
-            </sequence>
-        </complexType>
-    </element>
-    <complexType name="orderItemType">
-    	<attribute name="id" type="string" />
-    	<attribute name="quantity" type="int" />
-    </complexType>
-</schema>