chore: remove custom permissions feature flag definition, refs PM-20168 (#6551)

src/Core/Constants.cs

@@ -138,7 +138,6 @@ public static class FeatureFlagKeys
     public const string PolicyRequirements = "pm-14439-policy-requirements";
     public const string ScimInviteUserOptimization = "pm-16811-optimize-invite-user-flow-to-fail-fast";
     public const string EventBasedOrganizationIntegrations = "event-based-organization-integrations";
-    public const string SeparateCustomRolePermissions = "pm-19917-separate-custom-role-permissions";
     public const string CreateDefaultLocation = "pm-19467-create-default-location";
     public const string AutomaticConfirmUsers = "pm-19934-auto-confirm-organization-users";
     public const string PM23845_VNextApplicationCache = "pm-24957-refactor-memory-application-cache";