Add Overloaded removeMessageFromGroup to MGS

Currently, removeMessageFromGroup returns the message group. This requires loading the entire group from the backing store (when the store is persistent).

In many (all?) cases, the returned object is discarded (e.g. DelayHandler, AggregatingMessageHandler ).

Add an overloaded removeMessageFromGroup() that has a void (or boolean) return type.

This is particularly painful for large aggregation groups because the messages are removed by iteration (if expireGroupsOnCompletion is false).