Remove GeneratorToolsModernization feature flag (#5660)

Co-authored-by: Daniel James Smith <djsmith85@users.noreply.github.com>

Author: djsmith85

src/Core/Constants.cs

@@ -190,7 +190,6 @@ public static class FeatureFlagKeys
     public const string EnableRiskInsightsNotifications = "enable-risk-insights-notifications";
     public const string DesktopSendUIRefresh = "desktop-send-ui-refresh";
     public const string ExportAttachments = "export-attachments";
-    public const string GeneratorToolsModernization = "generator-tools-modernization";
 
     /* Vault Team */
     public const string PM8851_BrowserOnboardingNudge = "pm-8851-browser-onboarding-nudge";