Summary:
7358cd113ecbcb710e365c6ef8299fc7b72b9193 / D35385 refactored the bitmap scan tests, and mistakenly
removed the `bitmap_scans_distinct` test.

Add it back to the schedule, with a minor change to remove the `DIST` parameter from the queries, as
the number of requests changed between PG11 and PG15, but that isn't essential to the test.
Jira: DB-15622

Test Plan:
Jenkins: test regex: .*bitmap.*

```
./yb_build.sh --java-test 'org.yb.pgsql.TestPgRegressYbBitmapScans' -n 20
```

Reviewers: jason, tnayak

Reviewed By: jason

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D42299