Attempt to remove members from RegularImmutableMap when AdvacementUpdate packet processed on client

[Linguardium]

https://github.com/svenhjol/Charm/blob/master/src/main/java/svenhjol/charm/helper/AdvancementHelper.java#L34 https://github.com/svenhjol/Charm/blob/master/src/main/java/svenhjol/charm/helper/AdvancementHelper.java#L45

This code is run on both client and server. When the client receives the Advancement Update packet, it provides an immutable map. This line will fail on the clientside with an UnsupportedOperationException.

likely this causes #749