Remove obsolete column for DevOps analytics

What does this MR do and why?

Cleanups ignored column. It was ignored a while ago and never used since then.

DB migration

== 20211007093340 RemoveAnalyticsSnapshotsSegmentIdColumn: reverting ==========
-- add_column(:analytics_devops_adoption_snapshots, :segment_id, :bigint, {:after=>:id})
   -> 0.0321s
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:analytics_devops_adoption_snapshots)
   -> 0.0057s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE analytics_devops_adoption_snapshots\nADD CONSTRAINT fk_rails_25da9a92c0\nFOREIGN KEY (segment_id)\nREFERENCES analytics_devops_adoption_segments (id)\nON DELETE CASCADE\nNOT VALID;\n")
   -> 0.0040s
-- execute("ALTER TABLE analytics_devops_adoption_snapshots VALIDATE CONSTRAINT fk_rails_25da9a92c0;")
   -> 0.0041s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:analytics_devops_adoption_snapshots, [:segment_id, :end_time], {:name=>:index_on_snapshots_segment_id_end_time, :algorithm=>:concurrently})
   -> 0.0106s
-- add_index(:analytics_devops_adoption_snapshots, [:segment_id, :end_time], {:name=>:index_on_snapshots_segment_id_end_time, :algorithm=>:concurrently})
   -> 0.0078s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:analytics_devops_adoption_snapshots, [:segment_id, :recorded_at], {:name=>:index_on_snapshots_segment_id_recorded_at, :algorithm=>:concurrently})
   -> 0.0114s
-- add_index(:analytics_devops_adoption_snapshots, [:segment_id, :recorded_at], {:name=>:index_on_snapshots_segment_id_recorded_at, :algorithm=>:concurrently})
   -> 0.0076s
== 20211007093340 RemoveAnalyticsSnapshotsSegmentIdColumn: reverted (0.1338s) =

== 20211007093340 RemoveAnalyticsSnapshotsSegmentIdColumn: migrating ==========
-- remove_column(:analytics_devops_adoption_snapshots, :segment_id)
   -> 0.0082s
== 20211007093340 RemoveAnalyticsSnapshotsSegmentIdColumn: migrated (0.0082s) =

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