Remove cloud_license_enabled database column

What does this MR do?

Part of https://gitlab.com/gitlab-org/gitlab/-/issues/335811

After previously removing any references to the application setting's cloud_license_enabled and ignoring the column in the model, the database column can now be removed.

Up migration:

== 20210706112800 RemoveCloudLicenseEnabledFromApplicationSettings: migrating =
-- remove_column(:application_settings, :cloud_license_enabled)
   -> 0.0024s
== 20210706112800 RemoveCloudLicenseEnabledFromApplicationSettings: migrated (0.0092s) 

Down migration:

== 20210706112800 RemoveCloudLicenseEnabledFromApplicationSettings: reverting =
-- add_column(:application_settings, :cloud_license_enabled, :boolean, {:null=>false, :default=>false})
   -> 0.0030s
== 20210706112800 RemoveCloudLicenseEnabledFromApplicationSettings: reverted (0.0097s) 

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Does this MR meet the acceptance criteria?

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• I have self-reviewed this MR per code review guidelines.
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• This change is backwards compatible across updates, or this does not apply.

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