Remove unneeded index on packages_debian_{project,group}_architectures.distribution_id

What does this MR do?

See !51732 (comment 491491814)

Screenshots (strongly suggested)

Migration Up

== 20210419090412 RemoveDebianGroupArchitecturesDistributionIdIndex: migrating 
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:packages_debian_group_architectures, :distribution_id, {:name=>"idx_pkgs_deb_grp_architectures_on_distribution_id", :algorithm=>:concurrently})
   -> 0.0028s
-- remove_index(:packages_debian_group_architectures, {:name=>"idx_pkgs_deb_grp_architectures_on_distribution_id", :algorithm=>:concurrently, :column=>:distribution_id})
   -> 0.0061s
== 20210419090412 RemoveDebianGroupArchitecturesDistributionIdIndex: migrated (0.0102s) 

== 20210419090542 RemoveDebianProjectArchitecturesDistributionIdIndex: migrating 
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:packages_debian_project_architectures, :distribution_id, {:name=>"idx_pkgs_deb_proj_architectures_on_distribution_id", :algorithm=>:concurrently})
   -> 0.0026s
-- remove_index(:packages_debian_project_architectures, {:name=>"idx_pkgs_deb_proj_architectures_on_distribution_id", :algorithm=>:concurrently, :column=>:distribution_id})
   -> 0.0055s
== 20210419090542 RemoveDebianProjectArchitecturesDistributionIdIndex: migrated (0.0095s) 

Migration Down

== 20210419090542 RemoveDebianProjectArchitecturesDistributionIdIndex: reverting 
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:packages_debian_project_architectures, :distribution_id, {:name=>"idx_pkgs_deb_proj_architectures_on_distribution_id", :algorithm=>:concurrently})
   -> 0.0031s
-- execute("SET statement_timeout TO 0")
   -> 0.0010s
-- add_index(:packages_debian_project_architectures, :distribution_id, {:name=>"idx_pkgs_deb_proj_architectures_on_distribution_id", :algorithm=>:concurrently})
   -> 0.0161s
-- execute("RESET ALL")
   -> 0.0013s
== 20210419090542 RemoveDebianProjectArchitecturesDistributionIdIndex: reverted (0.0231s) 

== 20210419090412 RemoveDebianGroupArchitecturesDistributionIdIndex: reverting 
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:packages_debian_group_architectures, :distribution_id, {:name=>"idx_pkgs_deb_grp_architectures_on_distribution_id", :algorithm=>:concurrently})
   -> 0.0035s
-- add_index(:packages_debian_group_architectures, :distribution_id, {:name=>"idx_pkgs_deb_grp_architectures_on_distribution_id", :algorithm=>:concurrently})
   -> 0.0145s
== 20210419090412 RemoveDebianGroupArchitecturesDistributionIdIndex: reverted (0.0202s) 

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