fix: 删除demo文件

src/packages/step/demo.vue

-<template>
-  <div class="demo">
-    <h2>基本用法</h2>
-    <nut-steps> </nut-steps>
-  </div>
-</template>
-
-<script lang="ts">
-import { createComponent } from '@/utils/create';
-const { createDemo } = createComponent('steps');
-export default createDemo({
-  props: {},
-  setup() {
-    return {
-      // ...toRefs(state)
-    };
-  }
-});
-</script>
-
-<style lang="scss" scoped>
-.demo-box {
-}
-</style>