Fix double-loading of data in MITK FlowBench

Since gui.qt.flowapplication depends on gui.qt.ext, from which some code
code was initially copied to gui.qt.flowapplication and extended to load
data from command-line arguments, both routines for loading data were
called.

The mentioned extension (.mitksceneindex files) is now part of gui.qt.ext
and gui.qt.flowapplication no longer has a duplicate data loading routine.