[MP] Fix disconnect_peer not doing the proper cleanup

[Faless]

Lower layers (cache, replication) were not being notified about the peer being disconnected.

Fixes #91007 (note, there's another issue, causing a different error spam in the client, which should be fixed in a separate PR as it involves different codes).

Note: In this PR I'm preserving compatibility by not emitting the signal, I wonder if we want to change that, I see that there's some inconsistency across Godot when it comes to emitting signals in response to functions being called via code.

[grossqx]

Hi! Just built from a test branch where I merged this PR and this one too. When I kill the client process, server still gets a streak of 5 of this error. The error does not occur if client joins without a syncable character and leaves. But the amount of errors does not change when I change the sync interval.
You said in the other PR that this one might fix this issue, so I'm writing here. Let me know if I should create a separate issue for this after all. If you need any extra information, I'll be happy to test and report.

[DanielSnd]

When I kill the client process, server still gets a streak of 5 of this error.

This error you got after merging the PR looks like the error I got here: #92358
I made a change that fixed it for me, here's the pull request if you'd like to try it: #92359

[riazey]

Still getting this error in the 4.3 RC1 when deleting a multiplayer node with a synchronizer on it after a client has left, so I'm pretty sure this PR (or another) is still necessary alongside Daniel's fix!

[akien-mga]

Thanks!