housekeeping: Unconditionally enable writing multi-pack indices

We have introduced the logic to write multi-pack-indices as part of our repository housekeeping logic in v15.9. The initial rollout has caused some issues with pooled repositories, where we thought it was fine to write bitmaps when multi-pack-indices are enabled. But this turned out to still not be allowed and was subsequently fixed via c74a4b73 (housekeeping: Don't write bitmaps with MIDX in pooled repos, 2023-01-31). The subsequent rollout to production was successful and the feature flag was default-enabled in v15.9.

Remove the feature flag guarding this logic. This should allow us to more efficiently serve larger repositories that have many packfiles.

Closes #4741 (closed).