[REF] stock: remove `_product_available` method

This method is useless and it bypass the computation of some fields
(qty_available, virtual_available, incoming_qty, outgoing_qty). Then
this method is not cache-friendly, and we should use `read` instead.

task-2439019

closes odoo/odoo#66621

Related: odoo/enterprise#16584
Signed-off-by: Arnold Moyaux <amoyaux@users.noreply.github.com>