When using the Discuss app (not the systray's thread window) on a small
device, it is not possible to scroll through the discussion (cf. tab
"Chat" or "Channels").

Steps to reproduce:
* Go in discuss (in mobile)
* Go to tab chat or channel => BUG
NB: you must have enough element to test the scroll

This bug is due to the commit 3ad7706.

Before it, the styles as interpreted by the browser looked like this:

    .o_mail_discuss_tab_pane {
        overflow-x:scroll; // explicitly set in discuss.scss
        overflow-y:auto;   // implicit ; due to the overflow-x rule
    }

But by removing the overflow-x rule, the styles ware interpreted as:

    .o_mail_discuss_tab_pane {
        overflow-x:visible;   // implicit ; due to the absence of overflow-* rule
        overflow-y:visible;   // implicit ; due to the absence of overflow-* rule
    }

In addition, the parent element had an overflow: auto rule scoped only
for desktop-like screens.

This commit fixes this issue by unscoping the overflow: auto rule on the
parent, allowing to scroll the messages' list in both desktop-like and
mobile-like screens.

Reference:
https://developer.mozilla.org/en-US/docs/Web/CSS/overflow-x

Forward-Port-Of: odoo/odoo#45897