virsh: Remove --flags from nodesuspend

virsh: Remove --flags from nodesuspend

We always expose individual bits from flags as separate options rather
than exposing a raw flags options. Since virNodeSuspendForDuration does
not currently support any flags, the only way of using this --flags
options that would not fail is "--flags 0", which is equivalent to
omitting the option. Thus it is highly unlikely anyone would actually be
using it and removing it should be safe.