Re-remove firebase devDependency from rxfire (#3423)

* Re-remove firebase devDependency from rxfire

* Fix rollup version

Author: hsubox76

packages/rxfire/package.json

@@ -43,8 +43,7 @@
     "rxjs": "6.x.x"
   },
   "devDependencies": {
-    "firebase": "7.16.0",
-    "rollup": "2.7.6",
+    "rollup": "2.21.0",
     "rollup-plugin-commonjs": "10.1.0",
     "rollup-plugin-node-resolve": "5.2.0",
     "rollup-plugin-typescript2": "0.27.1",