Remove scenarioRefId from attribute group scenarioRefGrp

[kleihan]

Orchestra v1.1 RC3 removed numeric scenario identifiers (see #271). However, the numeric identifier for scenario references (scenarioRefId) was not removed. The attribute is part of the attribute group scenarioRefGrp, whereby only the name (scenarioRef) will be left. Hence, the group is no longer needed and can be replaced with the remaining attribute scenarioRef.

	<xs:attributeGroup name="scenarioRefGrp">
		<xs:annotation>
			<xs:documentation>A reference to a scenario by its key identifiers. There are no defaults as scenario references are optional.</xs:documentation>
		</xs:annotation>
		<xs:attribute name="scenarioRefId" type="fixr:id_t">
			<xs:annotation>
				<xs:documentation>Unique identifier of a scenario. The identifier is required when referencing another scenario.</xs:documentation>
			</xs:annotation>
		</xs:attribute>
		<xs:attribute name="scenarioRef" type="fixr:Name_t">
			<xs:annotation>
				<xs:documentation>Name of a scenario. The name is optional as part of a reference and only for convenience.</xs:documentation>
			</xs:annotation>
		</xs:attribute>
	</xs:attributeGroup>