refactor(clubSettings): remove myTischtennis club ID and update related logic

- Removed the myTischtennis club ID field from club settings and related backend logic.
- Updated the database migration to eliminate the my_tischtennis_club_id column.
- Adjusted frontend components and services to reflect the removal of the club ID, ensuring the use of association member number instead.
- Enhanced user interface hints to clarify the use of association member number for rankings.

backend/models/Club.js

@@ -17,12 +17,6 @@ const Club = sequelize.define('Club', {
     allowNull: true,
     field: 'association_member_number'
   },
-  myTischtennisClubId: {
-    type: DataTypes.STRING,
-    allowNull: true,
-    field: 'my_tischtennis_club_id',
-    comment: 'myTischtennis club number for rankings (e.g. 43030)'
-  },
   myTischtennisFedNickname: {
     type: DataTypes.STRING,
     allowNull: true,