Remove remenants of mob code

Since minetest has no mobs within the core anymore, I suppose these
settings and code should go. Any mod that uses `minetest.setting_getbool`
will work with no problem since the default return value is `false`.

src/defaultsettings.cpp

@@ -197,7 +197,6 @@ void set_default_settings(Settings *settings)
 	settings->setDefault("strict_protocol_version_checking", "false");
 	settings->setDefault("creative_mode", "false");
 	settings->setDefault("enable_damage", "true");
-	settings->setDefault("only_peaceful_mobs", "false");
 	settings->setDefault("fixed_map_seed", "");
 	settings->setDefault("give_initial_stuff", "false");
 	settings->setDefault("default_password", "");

src/environment.cpp

@@ -1225,11 +1225,6 @@ void ServerEnvironment::step(float dtime)
 				i != m_active_objects.end(); ++i)
 		{
 			ServerActiveObject* obj = i->second;
-			// Remove non-peaceful mobs on peaceful mode
-			if(g_settings->getBool("only_peaceful_mobs")){
-				if(!obj->isPeaceful())
-					obj->m_removed = true;
-			}
 			// Don't step if is to be removed or stored statically
 			if(obj->m_removed || obj->m_pending_deactivation)
 				continue;

src/serverobject.h

@@ -97,8 +97,6 @@ public:
 	// If object has moved less than this and data has not changed,
 	// saving to disk may be omitted
 	virtual float getMinimumSavedMovement();
-	
-	virtual bool isPeaceful(){return true;}
 
 	virtual std::string getDescription(){return "SAO";}