Remove docs-clean script (#2760)

Jekyll removes files by default

package.json

@@ -29,8 +29,7 @@
     "css-lint": "stylelint \"build/scss/**/*.scss\" --cache --cache-location .cache/.stylelintcache",
     "compile": "npm-run-all --parallel js css-all",
     "dev": "npm-run-all --parallel watch sync",
-    "docs": "npm-run-all --sequential docs-clean docs-prepare docs-compile",
-    "docs-clean": "cd docs/ && bundle exec jekyll clean -d ../docs_html",
+    "docs": "npm-run-all --sequential docs-prepare docs-compile",
     "docs-compile": "cd docs/ && bundle exec jekyll build -d ../docs_html",
     "docs-serve": "cd docs/ && bundle exec jekyll serve",
     "docs-prepare": "node build/npm/DocsPublish.js -v",