Remove tmp idx null project namespace id

What does this MR do and why?

Describe in detail what your merge request does and why.

#357409 (closed)

Remove temporary index tmp_index_for_null_project_namespace_id

% rails db:migrate
main: == 20220920022404 RemoveTempIndexNullProjectNamespaceId: migrating ============
main: -- transaction_open?()
main:    -> 0.0000s
main: -- indexes(:projects)
main:    -> 0.0459s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0009s
main: -- remove_index(:projects, {:algorithm=>:concurrently, :name=>"tmp_index_for_null_project_namespace_id"})
main:    -> 0.0046s
main: -- execute("RESET statement_timeout")
main:    -> 0.0004s
main: == 20220920022404 RemoveTempIndexNullProjectNamespaceId: migrated (0.0651s) ===


% rails db:rollback:main STEP=1
main: == 20220920022404 RemoveTempIndexNullProjectNamespaceId: reverting ============
main: -- transaction_open?()
main:    -> 0.0000s
main: -- index_exists?(:projects, :id, {:name=>"tmp_index_for_null_project_namespace_id", :where=>"project_namespace_id IS NULL", :algorithm=>:concurrently})
main:    -> 0.0199s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0003s
main: -- add_index(:projects, :id, {:name=>"tmp_index_for_null_project_namespace_id", :where=>"project_namespace_id IS NULL", :algorithm=>:concurrently})
main:    -> 0.0077s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: == 20220920022404 RemoveTempIndexNullProjectNamespaceId: reverted (0.0344s) ===

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