Squiz/FunctionDuplicateArgument: remove incorrect documentation

Looks like this XML file was committed/accepted by accident / was an unfinished copy/paste error, as it has nothing to do with the `Squiz.Functions.FunctionDuplicateArgument` sniff.

The sniff should be propely documented and the sniff has now been added to the task list in 148

Author: jrfnl