Tweak postgres indexing fix

[hubertdeng123]

Now fixes the groupedmessage indexing issue for a wider variety of users as reported in #2780, particularly the one related to this error:

ERROR:  cannot drop index sentry_groupedmessage_project_id_id_515aaa7e_uniq because constraint sentry_groupedmessage_project_id_id_515aaa7e_uniq on table sentry_groupedmessage requires it
HINT:  You can drop constraint sentry_groupedmessage_project_id_id_515aaa7e_uniq on table sentry_groupedmessage instead.