Multi is the default api for methods, it is not necessary to explicitly
decorate methods with it, adds clutter and most people use it because
they see that the rest of the code uses it.
Done with `find . -type f -name '*.py' | xargs sed -i '/@api.multi/d'`
The old tree views don't really exist anymore, this odd pseudo-flag to
dispatch between "list" and "tree" tree views has no reason to remain.
Task 1937686
closesodoo/odoo#31243
Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>