Add missing semicolons

Issue https://github.com/alphagov/govuk_frontend_toolkit/issues/146 brought up the fact that some semicolons were missing from some of the JavaScript files.

The intention of this pull request is to make these files more consistent with their author's use of semicolons. It is not a comment whether or how they *should* be used.

Note: this also includes the deletion of a duplicate variable that was also picked up.