[hooks/syncschema] avoid altering a dropped table
If the constraint's rdef is being removed, CWConstraintDelOp doesn't
need to do anything. Otherwise it may try to alter a removed
table/column. Closes #4154549
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))