[FIX] mail: text overlap on list view

Remove no longer needed `!important` on `overflow: visible` because the
new list view doesn't have any stacking context issue. Thanks to the
`o_PopoverManager` inside the `o-main-components-container`.

Steps to reproduce:

> Open the helpdesk app
> Click on VIP Support tickets
> Go to the list view
> reduce the size of the activities column
  => it's produce text contain overlapping with the other column => bug

X-original-commit: e22abffdc230c2ae494a85fdf7fd8d6b488be3ea
Part-of: odoo/odoo#102608

addons/mail/static/src/scss/mail_activity.scss

@@ -33,7 +33,6 @@
 .o_list_view, .o_legacy_list_view {
     .o_list_table tbody > tr {
         > td.o_data_cell.o_list_activity_cell {
-            overflow: visible !important; // allow the activity dropdown to overflow
             .o_mail_activity {
                 display: flex;
                 max-width: 275px;