refactor: Tree internals

BREAKING CHANGE: remove Deref/DrefMut to "TreeInterface", which
has been removed.

src/trees/tree.rs

@@ -1,6 +1,3 @@
-use std::ops::Deref;
-use std::ops::DerefMut;
-
 use crate::sys::bindings as ll_bindings;
 use crate::sys::{LLTree, TreeSequence};
 use crate::Position;
@@ -17,19 +14,6 @@ pub struct Tree<'treeseq> {
     advanced: i32,
 }
 
-impl Deref for Tree<'_> {
-    type Target = TreeInterface;
-    fn deref(&self) -> &Self::Target {
-        &self.api
-    }
-}
-
-impl DerefMut for Tree<'_> {
-    fn deref_mut(&mut self) -> &mut Self::Target {
-        &mut self.api
-    }
-}
-
 impl<'treeseq> Tree<'treeseq> {
     pub(crate) fn new<F: Into<TreeFlags>>(
         ts: &'treeseq crate::sys::TreeSequence,