DELETE /yoyo/credentials is entirely non-functional

[davidchatting-bot]

Where

Two related sites in `src/YoYoWiFiManager.cpp`:

`onYoYoRequestDELETE()` - the whole implementation is commented out, and it always returns 500:
```cpp
int YoYoWiFiManager::onYoYoRequestDELETE(uint8_t *data, size_t len, AsyncWebServerRequest *request) {
//TODO fix this!
...(all commented out)...
return(500);
}
```

`onYoYoMessageDELETE()` - even if the above were fixed and this got called, it doesn't actually do anything:
```cpp
if (message["path"] == "/yoyo/credentials") {
//TODO: implement delete using YoYoSettings::removeNetwork()
success = true;
}
```

Problem

So today, two independent bugs stack to make DELETE completely non-functional: the request never reaches `onYoYoMessageDELETE()` at all (every DELETE to any `/yoyo/*` path gets a 500), and even the handler for the one implemented path (`/yoyo/credentials`) just claims success without calling `YoYoSettings::removeNetwork()` - i.e. it's a no-op that lies about succeeding.

Ask

• Re-enable/fix `onYoYoRequestDELETE()`'s JSON body parsing (mirroring the pattern already used in `onYoYoRequestPOST()`, including the bounded-copy fix from Off-by-one buffer overflow parsing POST body in onYoYoRequestPOST() #57).
• Wire `onYoYoMessageDELETE()`'s `/yoyo/credentials` case up to `YoYoSettings::removeNetwork(ssid)`, which already exists and is otherwise unused from any endpoint.