MQE-593: Return variables should not have their own attributes

- Fixed one missing returnVariable on LoadSessionSnapshot.

Author: KevinBKozan

src/Magento/FunctionalTestingFramework/Test/etc/testSchema.xsd

@@ -835,7 +835,6 @@
         <xs:simpleContent>
             <xs:extension base="xs:string">
                 <xs:attribute type="xs:string" name="userInput"/>
-                <xs:attribute type="xs:string" name="returnVariable"/>
                 <xs:attribute type="xs:string" name="stepKey" use="required"/>
                 <xs:attribute type="xs:string" name="before"/>
                 <xs:attribute type="xs:string" name="after"/>