Drop an unused index_namespaces_on_runners_token

What does this MR do and why?

Drops the index index_namespaces_on_runners_token on namespaces, which is no longer used.

We can see that the index has seen zero scans in the past month on any database:

https://thanos-query.ops.gitlab.net/graph?g0.expr=sum%20by%20(type)(rate(pg_stat_user_indexes_idx_scan%7Benv%3D%22gprd%22%2C%20type%3D%22patroni%22%2C%20indexrelname%3D%22index_namespaces_on_runners_token%22%7D%5B30d%5D))&g0.tab=0&g0.stacked=0&g0.range_input=1h&g0.max_source_resolution=0s&g0.deduplicate=1&g0.partial_response=0&g0.store_matches=%5B%5D

We can also see that the namespaces.runners_token column is not used at all, as runner tokens are encrypted:

gitlabhq_dblab=# select id from namespaces where runners_token is not null;
 id 
----
(0 rows)

See #425979 (closed) for more details.

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Related to #425979 (closed)