Clean up commands.cc
The file ui/commands.cc contains a great number of methods, most of which are never used by Dune. Remove all unused methods, while making sure that test-ug, test-parallel-ug, and test-loadbalancing (from dune-grid) still build and run.
Note: the methods InitCommands in commands.cc is called from outside the file, but I think this call can be safely removed.