fix: re-add drop for old system_fronters view

831b6d3301 removed the view, and this line in
the cleanup SQL.

The DB migration for 6-char IDs fails to apply if this view exists - so even
though the view itself no longer exists in the code, we should always drop
it on cleanup.

PluralKit.Core/Database/clean.sql

@@ -3,6 +3,7 @@
 -- This does mean we can't use any functions in row triggers, etc. Still unsure how to handle this.
 
 drop view if exists system_last_switch;
+drop view if exists system_fronters;
 drop view if exists member_list;
 drop view if exists group_list;