perf: remove unused showdown

Co-authored-by: N花裤衩 <panfree23@gmail.com>

package.json

@@ -60,7 +60,6 @@
     "path-to-regexp": "2.4.0",
     "screenfull": "4.2.0",
     "script-loader": "0.7.2",
-    "showdown": "1.9.0",
     "sortablejs": "1.8.4",
     "tui-editor": "1.3.3",
     "vue": "2.6.10",