backend: Remove unused indexes

At present there are few indexes that  are never used, unused indexes
can affect insert performance so they are removed.

Signed-off-by: yolossn <santhosh@kinvolk.io>

pkg/api/db/migrations/0012_drop_unused_indexes.sql

@@ -0,0 +1,23 @@
+-- +migrate Up
+
+drop index if exists users_team_id_idx;
+
+drop index if exists instance_status_history_application_id_idx;
+
+drop index if exists event_event_type_id_idx;
+
+drop index if exists event_application_id_idx;
+
+drop index if exists channel_package_id_idx;
+
+-- +migrate Down
+
+create index if not exists users_team_id_idx on users (team_id);
+
+create index if not exists instance_status_history_application_id_idx on instance_status_history (application_id);
+
+create index if not exists event_event_type_id_idx on event (event_type_id);
+
+create index if not exists event_application_id_idx on event (application_id);
+
+create index if not exists channel_package_id_idx on channel (package_id);