clean up language filter: remove unneeded "if (!$this->default_lang)" line 66

Checking if the default_lang property is filled, was a reminiscense of those properties being static and the constructor only being called once. The comment 2 lines higher ("Ensure that constructor is called one time") belonged to this conditional. Now the properties are not static anymore and so the test is not necessary, as the constructor is only called once for every instantiation. 

To TEST this PR: multilanguage should be working exactly the same when applied, for no functionality is changed. Just removed a test that would always evaluate to true.

Comment: there is some deprecated code used:
* JApplication::getHash should now be JApplicationHelper::getHash
* JString should now be \Joomla\String\String

Maybe we shoud clean that up too in one go.