Improved performance of delete_session in case there is none

[thorsteneckel]

As stated in #130 this improves the performance of delete_session in case there is no existing session.

The delete_session method uses the get_session_model method which creates a new instance if no existing is found only to destroy it afterwards. Using @@session_class.find_by_session_id(id) instead (which is get_session_model doing too) works fine in our env and avoids the overhead.

[rails-bot]

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