Fire action accounts for all kinds of shoulder straps

I've noticed only "adjustable strap" works with the fire action to drawn
weapon, and only with default format (hanging like backpack). Happens
that code checks for that specific item. Solution is to check for a flag
instead, and so works with all kinds of attachments

src/character_attire.cpp

@@ -60,8 +60,6 @@ static const efftype_id effect_onfire( "onfire" );
 static const flag_id json_flag_HIDDEN( "HIDDEN" );
 static const flag_id json_flag_ONE_PER_LAYER( "ONE_PER_LAYER" );
 
-static const itype_id itype_shoulder_strap( "shoulder_strap" );
-
 static const material_id material_acidchitin( "acidchitin" );
 static const material_id material_bone( "bone" );
 static const material_id material_chitin( "chitin" );
@@ -2052,7 +2050,7 @@ void outfit::fire_options( Character &guy, std::vector<std::string> &options,
 
             actions.emplace_back( [&] { guy.invoke_item( &clothing, "holster" ); } );
 
-        } else if( clothing.is_gun() && clothing.gunmod_find( itype_shoulder_strap ) ) {
+        } else if( clothing.is_gun() && clothing.gunmod_find_by_flag( flag_BELTED ) ) {
             // wield item currently worn using shoulder strap
             options.push_back( clothing.display_name() );
             actions.emplace_back( [&] { guy.wield( clothing ); } );