[FIX] orm: better removal of custom m2m fields

orm: do not try to create ir.model.relation for custom m2m as self._module is either empty (for custom models), either the one of the last inheriting module (which is wrong). The field should be removed manually and should not be impacted by the uninstallation of modules. The removal of the relation table can be done when removing manually the custom field (see rev 6af3193).

ir.model: when removing a model, drop the table with the CASCADE instruction. This will remove left constraints from remaining m2m tables.
This means that dropping a table (either manually removing a custom model or uninstalling a module) will not drop the relation table for a custom m2m field. This is not ideal but better than the previous behaviour (which was to fail the DROP TABLE instruction and keep the table with a few columns and unconsistent data).