Explain the reason of an editor build target

### Godot version

Godot `4.6.1` packaged in Arch Linux

### godot-cpp

Version `10.0.0-rc1` albeit this goes with many versions back.

### System information

Arch Linux

### Issue description

I'm trying to do builds with multiple configurations, debug plus release, with CMake and Ninja. While I was on that I've noticed that there was an editor target which I assume that it's for enabling editor plugins or whatever. The point is that when I've looked up on what selecting it does is nothing besides setting a preprocessor definition named `TOOLS_ENABLED` which is used nowhere in the source code. I've grepped with and without taking into account `.gitignore` but it displays no usage of said definition. It might have been used some time ago but it's useless nowadays.

Haven't opened a PR to not jump the gun.

### Steps to reproduce

Grep the code for `TOOLS_ENABLED` or `editor` or something.

### Minimal reproduction project

Just reproduce what I've said in this repo.