From my point of view I dont see the point in emptying a string translation, when you can easily modify how the controller is working.
I guess this way is much more correct and satisfying.
A better answer could be to override destroy method in sessionController.
Just creates a file placed in: app/controllers/sessions_controller.rb
As you can see we comment the line creating the flash message.
class SessionsController < Devise::SessionsController
# DELETE /resource/sign_out
def destroy
signed_out = (Devise.sign_out_all_scopes ? sign_out : sign_out(resource_name))
#set_flash_message :notice, :signed_out if signed_out && is_flashing_format?
yield if block_given?
respond_to_on_destroy
end
end