Remove unneeded imports.

Remove unneeded imports.

* contrib/destroy_repository.py, cvs2svn_lib/changeset.py,
  cvs2svn_lib/changeset_graph.py, cvs2svn_lib/changeset_graph_node.py,
  cvs2svn_lib/checkout_internal.py, cvs2svn_lib/collect_data.py,
  cvs2svn_lib/common.py, cvs2svn_lib/cvs_file_database.py,
  cvs2svn_lib/cvs_item.py, cvs2svn_lib/cvs_item_database.py,
  cvs2svn_lib/cvs_revision_manager.py,
  cvs2svn_lib/dumpfile_delegate.py, cvs2svn_lib/fill_source.py,
  cvs2svn_lib/git_output_option.py,
  cvs2svn_lib/git_revision_recorder.py,
  cvs2svn_lib/metadata_database.py, cvs2svn_lib/openings_closings.py,
  cvs2svn_lib/passes.py, cvs2svn_lib/persistence_manager.py,
  cvs2svn_lib/process.py, cvs2svn_lib/project.py,
  cvs2svn_lib/property_setters.py,
  cvs2svn_lib/rcs_revision_manager.py,
  cvs2svn_lib/repository_delegate.py, cvs2svn_lib/run_options.py,
  cvs2svn_lib/stats_keeper.py, cvs2svn_lib/svn_commit.py,
  cvs2svn_lib/svn_commit_creator.py, cvs2svn_lib/svn_output_option.py,
  cvs2svn_lib/symbol_database.py, cvs2svn_lib/symbol_statistics.py,
  cvs2svn_lib/symbol_strategy.py: Remove unneeded imports.

git-svn-id: http://cvs2svn.tigris.org/svn/cvs2svn/trunk@4628 be7e6eca-30d4-0310-a8e5-ac0d63af7087