refactor(@angular-devkit/schematics): remove optimize from host-tree

Remove unused optimize method marked for removal in version 7.0

Author: simjes

packages/angular_devkit/schematics/src/tree/host-tree.ts

@@ -148,12 +148,6 @@ export class HostTree implements Tree {
     return this._record.willRename(path);
   }
 
-  // This can be used by old Schematics library with new Trees in some corner cases.
-  // TODO: remove this for 7.0
-  optimize() {
-    return this;
-  }
-
   branch(): Tree {
     const branchedTree = new HostTree(this._backend);
     branchedTree._record = this._record.clone();