Remove zen-observable (#509)

The `zen-observable` dependency was added by 516035a, but it seems unused.
Ref 516035a

Author: ybiquitous

package-lock.json

@@ -251,8 +251,7 @@
     "ts-jest": "^27.0.7",
     "ts-node": "^10.4.0",
     "typed-emitter": "^2.1.0",
-    "typescript": "^4.5.2",
-    "zen-observable": "^0.8.15"
+    "typescript": "^4.5.2"
   },
   "scripts": {
     "build": "tsc -p tsconfig.src.json",