Remove unnecesary conditional.

Remove unnecesary conditional.

The conditional is not just unnecessary, it would have been wrong.  The code
is suppose to be checking if the 'splitmirrors_ARG' is negative, but it
instead is checking 'mirrors_ARG'.  Rather than changing the argument being
checked, I've pulled the check entirely because 'splitmirrors_ARG' is already
guarenteed to not be negative by virtue of the fact that it is a 'int_arg'.
Negative values will be caught in _process_command_line().