Resolve "Remove the index_namespaces_id_parent_id_is_not_null index"

What does this MR do?

Remove the index_namespaces_id_parent_id_is_not_null index added in !61098 (merged). Now that the namespaces traversal_ids migration has finished we no longer need this index.

$ rails db:migrate:up:main VERSION=20220505053504
== 20220505053504 RemoveNamespacesIdParentIdInversePartialIndex: migrating ====
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:namespaces, :id, {:name=>"index_namespaces_id_parent_id_is_not_null", :algorithm=>:concurrently})
   -> 0.0261s
-- execute("SET statement_timeout TO 0")
   -> 0.0008s
-- remove_index(:namespaces, {:name=>"index_namespaces_id_parent_id_is_not_null", :algorithm=>:concurrently, :column=>:id})
   -> 0.0224s
-- execute("RESET statement_timeout")
   -> 0.0008s
== 20220505053504 RemoveNamespacesIdParentIdInversePartialIndex: migrated (0.0625s)

$ rails db:migrate:down:main VERSION=20220505053504
== 20220505053504 RemoveNamespacesIdParentIdInversePartialIndex: reverting ====
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:namespaces, :id, {:where=>"parent_id IS NOT NULL", :name=>"index_namespaces_id_parent_id_is_not_null", :algorithm=>:concurrently})
   -> 0.0254s
-- execute("SET statement_timeout TO 0")
   -> 0.0007s
-- add_index(:namespaces, :id, {:where=>"parent_id IS NOT NULL", :name=>"index_namespaces_id_parent_id_is_not_null", :algorithm=>:concurrently})
   -> 0.0048s
-- execute("RESET statement_timeout")
   -> 0.0008s
== 20220505053504 RemoveNamespacesIdParentIdInversePartialIndex: reverted (0.0449s)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
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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)
• I have followed the style guides.
• 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 #330099 (closed)