[FIX] mass_mailing: do not open link dialog with Powerbox

There are two ways of editing links: the link tools (in the sidebar) and
the link dialog. In `mass_mailing`, the link tools were in the sidebar
but using the link command of the Powerbox opened the link dialog. This
makes it so only one mechanism is in use (the link tools).

task-2856858

Part-of: odoo/odoo#91314