Remove leftover files, everything is now in base

The decimal precision feature makes sense to be an ORM feature, no need to be
in a specific module

Previous syntax was
    from odoo.addons import decimal_precision as dp
    fields.Float(digits=dp.get_precision('Foo'))

and now is:
    fields.Float(digits='Foo')

Remove the possibility to have a callable method on the digits attribute (it
was only used for precision anyway) and directly retrieve the digits on the
decimal.precision model

Rename the method digits to get_digits to avoid confusion between the field
attribute when declaring a field and the method to retrieve the precision

Task id: 48198

Had to alter marketing_campaign as it imported get_precision from deeper
into the module instead of properly getting it from the module's toplevel.