Remove long-unused internal plugin system

We are trying to update Express to fix a vulnerability.

We would have to update the plugins as well, but since we are no longer
using the plugin system, we can just delete it instead.

test/tsconfig.json

@@ -1,5 +1,4 @@
 {
   "extends": "../tsconfig.json",
-  "include": ["./**/*.ts"],
-  "exclude": ["./unit/node/test-plugin"]
+  "include": ["./**/*.ts"]
 }