[FIX] website_sale: clear old pricelist in the session

On the ecommerce, we want the pricelist to be adapted
to the user who is connected.
If no user is logged in, the 'Public Pricelist' is used.

To determine the pricelist that will be used (by default):
https://github.com/odoo/odoo/blob/a3169ede4f609b56c83da04756d20b1c7a07251f/addons/website_sale/controllers/main.py#L350-L354



Therefore, if we have been to the shop as a 'Public user' and we decide to log in,
we already have a pricelist in `request.session.get('website_sale_current_pl')`.

So we have to clear the pricelist when we connect to determine the new pricelist
of the user.

opw-3228998

closes odoo/odoo#118880

Signed-off-by: Lefebvre Thomas (thle) <thle@odoo.com>