Remove reference to scenarioRefId in Chapter 3.11.2

[kleihan]

Numeric scenario identifiers were removed with Orchestra v1.1 RC3. The specification was updated accordingly with the exception of the description of message scenarios:

A message may reference another scenario of the same message with the scenarioRefId and (optionally) scenarioRef attribute when it does not need to contain all of the elements of the referenced message scenario. It may contain the same elements, e.g. when one or more elements use a different scenario. The order of elements in the referencing scenario should be identical to the referenced scenario.

The part "scenarioRefId and (optionally)" needs to be removed, aligning it with the rest of the specification describing scenario references.