Resolve "Remove namespace id partial index"

What does this MR do?

We created a partial index on namespaces.id in order to iterate over the namespaces table as part of a data migration.

This MR removes the index now that it is no longer needed.

$ rails db:migrate:up:main VERSION=20220505060011                                                                                                                                                   == 20220505060011 RemoveNamespacesIdParentIdPartialIndex: migrating ===========
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:namespaces, :id, {:name=>"index_namespaces_id_parent_id_is_null", :algorithm=>:concurrently})
   -> 0.0304s
-- execute("SET statement_timeout TO 0")
   -> 0.0008s
-- remove_index(:namespaces, {:name=>"index_namespaces_id_parent_id_is_null", :algorithm=>:concurrently, :column=>:id})
   -> 0.0259s
-- execute("RESET statement_timeout")
   -> 0.0007s
== 20220505060011 RemoveNamespacesIdParentIdPartialIndex: migrated (0.0707s) ==

$ rails db:migrate:down:main VERSION=20220505060011                                                                                                                                                 == 20220505060011 RemoveNamespacesIdParentIdPartialIndex: reverting ===========
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:namespaces, :id, {:where=>"parent_id IS NULL", :name=>"index_namespaces_id_parent_id_is_null", :algorithm=>:concurrently})
   -> 0.0200s
-- execute("SET statement_timeout TO 0")
   -> 0.0007s
-- add_index(:namespaces, :id, {:where=>"parent_id IS NULL", :name=>"index_namespaces_id_parent_id_is_null", :algorithm=>:concurrently})
   -> 0.0053s
-- execute("RESET statement_timeout")
   -> 0.0006s
== 20220505060011 RemoveNamespacesIdParentIdPartialIndex: reverted (0.0354s) ==

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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

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• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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Closes #327098 (closed)