[BUG] Invalid option GLOBAL_MESH_Z_OFFSET in bugfix 2.1.x branch

[ellensp]

Bug Description

Example configs for bugfix-2.1.x all contain the following in Configuration.h

  /**
   * Add Z offset (M424 Z) that applies to all moves at the planner level.
   * This Z offset will be automatically set to the middle value with G29.
   */
  //#define GLOBAL_MESH_Z_OFFSET

The code from 2020 for this has not yet been merged https://github.com/MarlinFirmware/Marlin/pull/18987

This is causing much confusion in users who try this option, and it doesn't do anything.

Steps to Reproduce

1. Look at any example config files in bugfix 2.1.x

Expected behavior:

Only valid configuration options are in example Config files

Actual behavior:

Invalid configuration options are present.

Additional Information

These are also present in import branch, but import branch can have things that are up and coming, but bugfix branch should not.