Small cleanup in 'set page'

Currently on every call to [`set page`](https://github.com/mozilla/pdf.js/blob/master/web/viewer.js#L298) a reference to the pageNumber input element is fetched from the DOM, even though this not used anywhere in the function after that. From using git blame, it appears that this hasn't been used since the custom `pagechange` event was introduced.

This PR simply removes what is now a meaningless piece of code. Since this patch is so small, I also took the liberty of fixing a few small coding style issues (braces for single line statements and strict equalities) in `set page`.