abc9_ops -prep_box: Adjust for repeated invocation

`abc9_ops -prep_box` command interprets the `abc9_box` attribute and
prepares a .box file for ABC consumption. Previously this command was
removing the attribute as it was processing each module which prevented
repeated invocation of this command unless the box definitions were
refreshed from a source file.

Also the command was keeping existing `abc9_box_id` attributes instead
of overwriting them with values from a new number sequence.

Change both behaviors to allow repeated invocations of the command on
the same design.

Author: povik

passes/techmap/abc9_ops.cc

@@ -969,13 +969,10 @@ void prep_box(RTLIL::Design *design)
 		if (it == module->attributes.end())
 			continue;
 		bool box = it->second.as_bool();
-		module->attributes.erase(it);
 		if (!box)
 			continue;
 
 		auto r = module->attributes.insert(ID::abc9_box_id);
-		if (!r.second)
-			continue;
 		r.first->second = abc9_box_id++;
 
 		if (module->get_bool_attribute(ID::abc9_flop)) {