Drop unused scope on ci builds

Ref: #271620 (closed)

What does this MR do and why?

This MR drops an unused scope on ci_builds.

In a follow up MR we will drop an unused index index_ci_builds_on_queued_at currently taking a lot of disk space.

List of relations  
 Schema |             Name             | Type  |  Owner  |   Table   | Size  | Description   
--------+------------------------------+-------+---------+-----------+-------+-------------  
 public | index_ci_builds_on_queued_at | index | joe_292 | ci_builds | 69 GB |   
(1 row)  

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