move multiplex plugin out to reveal/reveal-multiplex repo

package.json

@@ -30,7 +30,6 @@
     "babel-eslint": "^10.1.0",
     "colors": "^1.4.0",
     "core-js": "^3.6.5",
-    "express": "^4.17.1",
     "glob": "^7.1.6",
     "gulp": "^4.0.2",
     "gulp-autoprefixer": "^7.0.1",
@@ -41,13 +40,11 @@
     "gulp-sass": "^4.0.2",
     "gulp-tap": "^2.0.0",
     "gulp-zip": "^5.0.1",
-    "mustache": "^4.0.0",
     "node-qunit-puppeteer": "^2.0.1",
     "qunit": "^2.9.3",
     "rollup": "^2.6.1",
     "rollup-plugin-babel": "^4.4.0",
     "rollup-plugin-terser": "^5.3.0",
-    "socket.io": "^2.3.0",
     "yargs": "^15.1.0"
   },
   "browserslist": "> 0.5%, IE 11, not dead",