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.

ci/dev/test-unit.sh

@@ -6,15 +6,10 @@ main() {
 
   source ./ci/lib.sh
 
-  echo "Building test plugin"
-  pushd test/unit/node/test-plugin
-  make -s out/index.js
-  popd
-
   # We must keep jest in a sub-directory. See ../../test/package.json for more
   # information. We must also run it from the root otherwise coverage will not
   # include our source files.
-  CS_DISABLE_PLUGINS=true ./test/node_modules/.bin/jest "$@" --testRegex "./test/unit/.*ts" --testPathIgnorePatterns "./test/unit/node/test-plugin"
+  ./test/node_modules/.bin/jest "$@" --testRegex "./test/unit/.*ts"
 }
 
 main "$@"