[multi-sources-removal] Drop the "true" multi-sources planner
Only keep copy-based sources. This will soon allow for greater code
simplification.
Related to #2919300
import ConfigParser
try:
undo_actions = config.cfgfile_parser.get('MAIN', 'undo-support', False)
except ConfigParser.NoOptionError:
pass # this conf. file was probably already migrated
else:
config.global_set_option('undo-enabled', bool(undo_actions))