content/static/js: remove used .js-filesOption reference
content/static/js: remove used .js-filesOption reference

At the moment, this error appears in the console on pages without a
Source Files section:

Uncaught TypeError: Cannot read property 'setAttribute' of null
    at unit.js:54

It doesn't seem like js-filesOption is being used anywhere, so this
reference is deleted in unit.js.

Change-Id: I8db74ae996d99b1ffef143e5b8ffdc78ba1d9cbe
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/270801
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite