Remove the JSON decoder global override to convert _msat fields to Millisatoshi

[rustyrussell]

Now they're all integers (since 23.05, though some were missed and fxied in 23.08) we can just leave them! This means clnrest does not have to do strange things (see #6828), as we can remove the global patch for JSONDecoder!

We should still have the global JSON encoder handle Millisatoshi() because that's nice!