Remove sep max_concurrency application setting

What does this MR do and why?

In MR !174596 (merged), we updated our concurrency strategy to distribute the pipelines created by scan execution policies scheduled scans. The new strategy does not use the security_policy_scheduled_scans_max_concurrency application setting.

Since this application setting is not used anymore, it should be removed as suggested here.

Migration up

main: == 20250318172846 RemoveSecurityPolicyScheduledScansMaxConcurrencyFromApplicationSettings: migrating 
main: -- remove_column(:application_settings, :security_policy_scheduled_scans_max_concurrency)
main:    -> 0.0035s
main: == 20250318172846 RemoveSecurityPolicyScheduledScansMaxConcurrencyFromApplicationSettings: migrated (0.0079s) 

Migration down

main: == 20250318172846 RemoveSecurityPolicyScheduledScansMaxConcurrencyFromApplicationSettings: reverting 
main: -- add_column(:application_settings, :security_policy_scheduled_scans_max_concurrency, :integer, {:default=>100, :null=>false, :if_not_exists=>true})
main:    -> 0.2514s
main: == 20250318172846 RemoveSecurityPolicyScheduledScansMaxConcurrencyFromApplicationSettings: reverted (0.2592s) 

References

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• Related to #508142

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