Remove redundant assertion.
Remove redundant assertion.

set_irn_link checks whether the resource is allocated.

jonashaag/libfirm,MatzeB/libfirm,libfirm/libfirm,MatzeB/libfirm,libfirm/libfirm,jonashaag/libfirm,MatzeB/libfirm,libfirm/libfirm,libfirm/libfirm,jonashaag/libfirm,MatzeB/libfirm,MatzeB/libfirm,jonashaag/libfirm,libfirm/libfirm,MatzeB/libfirm,jonashaag/libfirm,jonashaag/libfirm,MatzeB/libfirm,jonashaag/libfirm