Before this the `--unaccent` flag does double duty to specify whether
to create new databases with the extension *and* to try to use the
corresponding function.

The latter is further gated on the function existing at all in the
database.

Given postgresql does not have unaccent installed by default it seems
only the latter is really useful and we can ignore the flag to decide
whether to enable unaccent features, if the extension is installed
consider that it should be enabled and move on.

Merge this in master rather than previous versions as it can change
things like index access / use.

closes odoo/odoo#47377

Signed-off-by: Xavier Morel (xmo) <xmo@odoo.com>