Fix dotnet_sort_system_directives_first to not specify severity
Fix dotnet_sort_system_directives_first to not specify severity

The option gets ignored when a severity is specified.
wtgodbe/corefx,shimingsg/corefx,ViktorHofer/corefx,wtgodbe/corefx,wtgodbe/corefx,shimingsg/corefx,ericstj/corefx,shimingsg/corefx,ericstj/corefx,wtgodbe/corefx,ViktorHofer/corefx,ViktorHofer/corefx,shimingsg/corefx,shimingsg/corefx,wtgodbe/corefx,ericstj/corefx,ericstj/corefx,shimingsg/corefx,shimingsg/corefx,ViktorHofer/corefx,ViktorHofer/corefx,ViktorHofer/corefx,ericstj/corefx,ViktorHofer/corefx,ericstj/corefx,wtgodbe/corefx,wtgodbe/corefx,ericstj/corefx