postgres: drop redundant indices (#5715)

## Summary
There are 3 indices in the postgres storage driver that are redundant.
This PR drops them.

## Related issues
-
[ENG-2560](https://linear.app/pomerium/issue/ENG-2560/request-remove-redundant-database-indexes)

## Checklist
- [x] reference any related issues
- [ ] updated unit tests
- [x] add appropriate label (`enhancement`, `bug`, `breaking`,
`dependencies`, `ci`)
- [x] ready for review

pkg/storage/postgres/migrate.go

@@ -156,6 +156,21 @@ var migrations = []func(context.Context, pgx.Tx) error{
 			}
 		}
 
+		return nil
+	},
+	6: func(ctx context.Context, tx pgx.Tx) error {
+		// these indices are redundant
+		for _, q := range []string{
+			`DROP INDEX ` + schemaName + `.` + recordsTableName + `_type_idx`,
+			`DROP INDEX ` + schemaName + `.` + recordChangesTableName + `_version_idx`,
+			`DROP INDEX ` + schemaName + `.` + recordChangesTableName + `_type_idx`,
+		} {
+			_, err := tx.Exec(ctx, q)
+			if err != nil {
+				return err
+			}
+		}
+
 		return nil
 	},
 }