MDL-58332 message: remove concat in get_popup_notifications sql
Removed the concat to generate the uniqueid field for the popup
notifications data. The concat can't be used directly in the SQL because
the syntax changes between databases. The sql_concat helper can't be
used because it assumes all values are database columns (which they
aren't in this case).
Instead I've just removed the uniqueid field because it isn't required
for the union all to work and the field isn't being used by anything.
This should fixed the compatibility issues between databases.
