Dropping unused variable in dropdown.js

Dropping an unused variable in the Dropdown function inside the `dropdown.js` code.

There are no new/changed unit tests, as there are no feature/bug changes requiring new test cases.

For the record: I agree to dual-license this contribution under the Apache 2 and MIT licenses.
Merge pull request #11709 from mdxs/patch-1

Dropping unused variable in dropdown.js