[MM-53086] Remove SendWelcomePost A/B test and feature (#23733)

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/app/user_test.go

@@ -1875,57 +1875,6 @@ func TestCreateUserWithInitialPreferences(t *testing.T) {
 	})
 }
 
-func TestIsFirstAdmin(t *testing.T) {
-	t.Run("should return false if user is not sysadmin", func(t *testing.T) {
-		th := SetupWithStoreMock(t)
-		defer th.TearDown()
-
-		Id := model.NewId()
-		isFirstAdmin := th.App.IsFirstAdmin(&model.User{
-			Id:    Id,
-			Roles: model.SystemUserRoleId,
-		})
-		require.False(t, isFirstAdmin)
-	})
-
-	t.Run("should return false if user is sysadmin but not the first one", func(t *testing.T) {
-		th := SetupWithStoreMock(t)
-		defer th.TearDown()
-
-		Id := model.NewId()
-
-		mockUserStore := storemocks.UserStore{}
-		mockUserStore.On("GetFirstSystemAdminID").Return(model.NewId(), nil)
-
-		mockStore := th.App.Srv().Store().(*storemocks.Store)
-		mockStore.On("User").Return(&mockUserStore)
-
-		isFirstAdmin := th.App.IsFirstAdmin(&model.User{
-			Id:    Id,
-			Roles: model.SystemAdminRoleId,
-		})
-		require.False(t, isFirstAdmin)
-	})
-
-	t.Run("should return true if user is sysadmin and the first one", func(t *testing.T) {
-		th := SetupWithStoreMock(t)
-		defer th.TearDown()
-
-		Id := model.NewId()
-
-		mockStore := th.App.Srv().Store().(*storemocks.Store)
-		mockUserStore := storemocks.UserStore{}
-		mockUserStore.On("GetFirstSystemAdminID").Return(Id, nil)
-		mockStore.On("User").Return(&mockUserStore)
-
-		isFirstAdmin := th.App.IsFirstAdmin(&model.User{
-			Id:    Id,
-			Roles: model.SystemAdminRoleId,
-		})
-		require.True(t, isFirstAdmin)
-	})
-}
-
 func TestSendSubscriptionHistoryEvent(t *testing.T) {
 	cloudProduct := &model.Product{
 		ID:                "prod_test1",