How to disable "You have been assigned" email when a user creates a sales order in Odoo 10

Overwrite _message_auto_subscribe_notify method for sale.order class and add to context mail_auto_subscribe_no_notify.

from odoo import models, api


class SaleOrder(models.Model):
    _inherit = 'sale.order'

    @api.multi
    def _message_auto_subscribe_notify(self, partner_ids):
        """ Notify newly subscribed followers of the last posted message.
            :param partner_ids : the list of partner to add as needaction partner of the last message
                                 (This excludes the current partner)
        """
        return super(SaleOrder, self.with_context(mail_auto_subscribe_no_notify=True))\
                ._message_auto_subscribe_notify(partner_ids)

The original method will not send the message if that key is passed in the context

    @api.multi
    def _message_auto_subscribe_notify(self, partner_ids):
        """ Notify newly subscribed followers of the last posted message.
            :param partner_ids : the list of partner to add as needaction partner of the last message
                                 (This excludes the current partner)
        """
        if not partner_ids:
            return

        if self.env.context.get('mail_auto_subscribe_no_notify'):  # Here
            return

        # send the email only to the current record and not all the ids matching active_domain !
        # by default, send_mail for mass_mail use the active_domain instead of active_ids.
        if 'active_domain' in self.env.context:
            ctx = dict(self.env.context)
            ctx.pop('active_domain')
            self = self.with_context(ctx)

        for record in self:
            record.message_post_with_view(
                'mail.message_user_assigned',
                composition_mode='mass_mail',
                partner_ids=[(4, pid) for pid in partner_ids],
                auto_delete=True,
                auto_delete_message=True,
                parent_id=False, # override accidental context defaults
                subtype_id=self.env.ref('mail.mt_note').id)