[#48056017] Uncomment es.indices.delete_index()

Indices older than the cutoff date were not being deleted (despite the log
line suggesting so) because the call to `delete_index()` was commented out.
I suspect this was commented while testing and then missed during the
commit.
Merge pull request #1 from alphagov/48056017-delete_old_indices

[#48056017] Uncomment es.indices.delete_index()