rust: Remove the compile-time 'dirstate-tree' feature flag This code has compiler errors since it is not built on CI and nobody has been working on it for some time. We (Octobus) are still pursuing status optimizations based on a tree data structure for the dirstate, but upcoming patches will use a run-time opt-in instead of compile-time, so that at least corresponding Rust code keeps compiling when other changes are made. Differential Revision: https://phab.mercurial-scm.org/D10329