7.0 uninstall module opw 605264 mat

[FIX] uninstall: avoid errors during uninstall (opw 605264)

When uninstalling a module, remove the ir.model.constraint after removing the non-model records and before fields and model definition.
Without this fix, some constraint would be removed too early allowing to have broken relations and data left from removed module.

Step to reproduce:
  1. start with a fresh database with mail module installed
  2. uninstall mail
  3. try to reinstall
-> error