feat: remove 'mutable-model' feature

[woshilapin]

This feature, that was introduced for an experimentation, didn't prove itself safe and constrained enough. Let's remove it.

Original PR was #761.

For details, opening this door of mutability was opening an opportunity to make the Model inconsistent. The consequence of that would be disastrous, since relations in the graph might become incorrect, the information incoherent and raise subtle bugs that might be hard to debug and fix.

Another problem with this modification, we modify the Model after it has been sanitized. But creating new Calendar was added, possibly connected, but possibly not (because other API for connecting), which means the Model probably ends up with orphan Calendar that should be sanitized (even if connected, the one that has been replaced might have become an orphan).

[ArnaudOggy]

@woshilapin Ok for me. Feel free to merge.

[pbougue]

OK to remove it, I think a bit more sharing on why it was added (maybe just a link to first PR) and more interesting why it's not worth keeping it would be nice.

[woshilapin]

I improved a bit the description. But to be honest, I vaguely remember finding an example of using this API and breaking the Model but it was a long time ago.