Refactor removeUnusedNS plugin (#1559)

- covered with types
- migrated to visitor plugin api
- dropped traverse utility which is replaced by visitor

lib/xast.js

@@ -55,26 +55,6 @@ const closestByName = (node, name) => {
 };
 exports.closestByName = closestByName;
 
-const traverseBreak = Symbol();
-exports.traverseBreak = traverseBreak;
-
-/**
- * @type {(node: any, fn: any) => any}
- */
-const traverse = (node, fn) => {
-  if (fn(node) === traverseBreak) {
-    return traverseBreak;
-  }
-  if (node.type === 'root' || node.type === 'element') {
-    for (const child of node.children) {
-      if (traverse(child, fn) === traverseBreak) {
-        return traverseBreak;
-      }
-    }
-  }
-};
-exports.traverse = traverse;
-
 const visitSkip = Symbol();
 exports.visitSkip = visitSkip;