Apply 'no-unnecessary-initializer' lint rule (#22014)

* Apply 'no-unnecessary-initializer' lint rule

Forbids `let`/`const` statements to be initialized to `undefined`, since that's the initial value by default anyway.
The auto-fixer also happened to remove two unnecessary `as number` casts in `src/harness/parallel/worker.ts`.

For historical data: to run with `--fix`, I modified the line in `Jakefile.js` that declared the `cmd` for running TSLint.

* Moved worker.ts type assertions to parameters

tslint.json

@@ -104,7 +104,6 @@
         "no-object-literal-type-assertion": false,
         "no-shadowed-variable": false,
         "no-submodule-imports": false,
-        "no-unnecessary-initializer": false,
         "no-var-requires": false,
         "ordered-imports": false,
         "prefer-conditional-expression": false,