Remove mobx dependency

[alvrs]

MUD (especially recs) started out building heavily on mobx as foundation for reactivity, but at some point switched to rxjs for performance reasons. However, mobx is still used in some places of the codebase, leading to "glue code" to convert mobx observable into rxjs observables and vice versa. We should remove all usages of mobx (eg replace them with rxjs's BehaviorSubject) to be able to remove mobx as a dependency.