[FIX] stock: correctly compute custom search method
[FIX] stock: correctly compute custom search method

The search on qty_available was refactored twice:
- b9853db8 to split the method
- a989680b to remove the eval operator

At b9853db8 the order of evaluation introduced a bug so that the domain
   ('qty_available', '=' 10.0)
was evaluated
   if product[qty_available] = 10.0:
which crashed (`=` instead of `==`).

Before b9853db8, this code did not crash as the operator was converted via
  if operator == '=':
    operator = '=='
which is no longer executed after the split

a989680b replaced the eval by a dictionnary that replaced the eval error by a
KeyError when searching with `=` operator.

This commit changes several things:
- replace the `operator in ('==', '>=', '<=')` as this was evaluated before the
  operator replacement code mentionned above
- remove the replacement code as no longer justified since a989680b
- replace the OPERATOR key `==` to always match given key

ygol/odoo,ygol/odoo,ygol/odoo,dfang/odoo,hip-odoo/odoo,hip-odoo/odoo,ygol/odoo,dfang/odoo,dfang/odoo,dfang/odoo,hip-odoo/odoo,dfang/odoo,hip-odoo/odoo,dfang/odoo,ygol/odoo,ygol/odoo,hip-odoo/odoo,hip-odoo/odoo,ygol/odoo