[PM-24554] Remove code for pm-20322-allow-trial-length-0 (#6220)

* [PM-24554] remove code for feature flag

* remove unused using

src/Core/Constants.cs

@@ -154,7 +154,6 @@ public static class FeatureFlagKeys
     public const string PM19422_AllowAutomaticTaxUpdates = "pm-19422-allow-automatic-tax-updates";
     public const string PM199566_UpdateMSPToChargeAutomatically = "pm-199566-update-msp-to-charge-automatically";
     public const string UseOrganizationWarningsService = "use-organization-warnings-service";
-    public const string PM20322_AllowTrialLength0 = "pm-20322-allow-trial-length-0";
     public const string PM21092_SetNonUSBusinessUseToReverseCharge = "pm-21092-set-non-us-business-use-to-reverse-charge";
     public const string PM21881_ManagePaymentDetailsOutsideCheckout = "pm-21881-manage-payment-details-outside-checkout";
     public const string PM21821_ProviderPortalTakeover = "pm-21821-provider-portal-takeover";

src/Identity/Billing/Controller/AccountsController.cs

@@ -1,7 +1,5 @@
-using Bit.Core;
-using Bit.Core.Billing.Models.Api.Requests.Accounts;
+using Bit.Core.Billing.Models.Api.Requests.Accounts;
 using Bit.Core.Billing.TrialInitiation.Registration;
-using Bit.Core.Services;
 using Bit.Core.Utilities;
 using Bit.SharedWeb.Utilities;
 using Microsoft.AspNetCore.Mvc;
@@ -11,16 +9,13 @@ namespace Bit.Identity.Billing.Controller;
 [Route("accounts")]
 [ExceptionHandlerFilter]
 public class AccountsController(
-    ISendTrialInitiationEmailForRegistrationCommand sendTrialInitiationEmailForRegistrationCommand,
-    IFeatureService featureService) : Microsoft.AspNetCore.Mvc.Controller
+    ISendTrialInitiationEmailForRegistrationCommand sendTrialInitiationEmailForRegistrationCommand) : Microsoft.AspNetCore.Mvc.Controller
 {
     [HttpPost("trial/send-verification-email")]
     [SelfHosted(NotSelfHostedOnly = true)]
     public async Task<IActionResult> PostTrialInitiationSendVerificationEmailAsync([FromBody] TrialSendVerificationEmailRequestModel model)
     {
-        var allowTrialLength0 = featureService.IsEnabled(FeatureFlagKeys.PM20322_AllowTrialLength0);
-
-        var trialLength = allowTrialLength0 ? model.TrialLength ?? 7 : 7;
+        var trialLength = model.TrialLength ?? 7;
 
         var token = await sendTrialInitiationEmailForRegistrationCommand.Handle(
             model.Email,