Avoid scanning entire module if there are no wires to remove

It's pretty common for `opt_clean` to find no wires to remove. In that case,
there is no point scanning the entire design, which can be significantly
expensive for huge designs.

kernel/rtlil.cc

@@ -2990,6 +2990,8 @@ void RTLIL::Module::add(RTLIL::Binding *binding)
 void RTLIL::Module::remove(const pool<RTLIL::Wire*> &wires)
 {
 	log_assert(refcount_wires_ == 0);
+	if (wires.empty())
+		return;
 
 	struct DeleteWireWorker
 	{