core: Remove GetStorageSessionsListQuery

GetStorageSessionsListQuery is not used anywhere in the code, and
hence should be removed.

This patch removes the following:
1. The GetStorageSessionsListQuery class
2. The GetStorageSessionsList constant representing it in
   VdcQueryType
3. The GetStorageSessionsListVDSCommand class which was only used by
   this query.
4. The GetStorageSessionList constant representing it in
   VDSCommandType.
5. The getSessionsList() method
   IVdsServer/VdsServerConnector/VdsServerWrapper, which was only used
   by GetStorageSessionsListVDSCommand.
6. The SessionsListReturnForXmlRpc class which was only used by
   GetStorageSessionsListVDSCommand.

Change-Id: I5ee43140a7ca0cbe9a2c15cc787c270ead619bda
Signed-off-by: Allon Mureinik <amureini@redhat.com>