Remove unused imports.

cobbler/collection_manager.py:28: 'distro' imported but unused
cobbler/collection_manager.py:29: 'profile' imported but unused
cobbler/collection_manager.py:30: 'system' imported but unused
cobbler/collection_manager.py:31: 'repo' imported but unused
cobbler/collection_manager.py:32: 'image' imported but unused
cobbler/collection_manager.py:33: 'mgmtclass' imported but unused
cobbler/collection_manager.py:34: 'package' imported but unused
cobbler/collection_manager.py:35: 'file' imported but unused