Delete an unnecessary assignment statement from parse_arg()

[elfring]

The source code analysis tool “Cppcheck 2.9-1.2” points the information “Variable 'opt' is assigned a value that is never used.” out. Thus the affected assignment statement can be deleted from the implementation of the function “parse_arg”.

[rewolff]

committed. (in my private copy: you won't see it yet. It will be pushed eventually).