Remove seat_link_enabled from ApplicationSettings db table

What does this MR do?

Part of https://gitlab.com/gitlab-org/gitlab/-/issues/332811

Prior to this change, the column seat_link_enabled in ApplicationSettings was ignored (!64160 (merged)) in the previous milestone. In the current milestone, the column can be dropped from the database.

Up migration:

== 20210721122840 RemoveSeatLinkEnabledFromApplicationSettings: migrating =====
-- remove_column(:application_settings, :seat_link_enabled)
   -> 0.0148s
== 20210721122840 RemoveSeatLinkEnabledFromApplicationSettings: migrated (0.0237s) 

Down migration:

== 20210721122840 RemoveSeatLinkEnabledFromApplicationSettings: reverting =====
-- add_column(:application_settings, :seat_link_enabled, :boolean, {:null=>false, :default=>true})
   -> 0.0065s
== 20210721122840 RemoveSeatLinkEnabledFromApplicationSettings: reverted (0.0155s) 

Does this MR meet the acceptance criteria?

Conformity

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• 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.

Availability and Testing

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