Remove SimplyEdit Backend as it is not used

[Potherca]

The full tree of this repository looks like this:

├── lib/
└── www/
    ├── api/
    │   └── data/
    ├── css/
    ├── data/
    ├── js/
    ├── simply-edit/
    └── templates/

The part we are currently interested in is where HTTP requests go:

  ├── lib/
  │   ├── config.php ─────────╮   ◀───╮
  │   ├── filesystem.php  ◀───┤       │
  │   ├── http.php        ◀───╯       │
  │   └── router.php                  │ require_once
  └── www/                            │
      ├── api/          ─╖            │
      │   ├── data/      ║ .htaccess  │
      │   └── index.php ─╨────────────╯
      ├── simply-edit/      ──╖
      │   ├── config.php      ║
      │   ├── filesystem.php  ║
      │   ├── http.php        ║ .htaccess
      │   ├── index.php       ║
      │   ├── router.php  ◁───╢
      │   └── store.php       ║
      ├── templates/        ──╜
      ├── generated.html -> api/data/generated.html
      └── index.html

Anything under www/ would normally be handled by the index.php (for /) or router.php (for everything else).

But the index.html overwrites the index.php and api/ has its own .htaccess.

So it looks like the only config, filesystem, and router actually in use are those in lib/.