If you are on CS:GO the engine provides you with cleanup stuff. (in case this isn't enough you can still use the plugin)
Code:
weapon_auto_cleanup_time - If set to non-zero, weapons will delete themselves after the specified time (in seconds) if no players are near
weapon_max_before_cleanup - If set to non-zero, will remove the oldest dropped weapon to maintain the specified number of dropped weapons in the world.