[cleanup][test][porenetwork] removing setPreviousSolution call

There are only three tests that use a function call in the form of assembler->setPreviousSolution(xOld). If this line is deleted and the tests compiled again, they perform exactly in the same way as before, therefore I would suggest, to remove this call as access to the old solution is already given when the constructor of the assembler is called.

This raises the question, whether this setPreviousSolution() method shall be deleted completely, as it is not called anywhere else except those three tests, or if it is maybe left as it could be used under other circumstances