Deprecate pseudo user styles code

The pseudo user styles code served only browsers based
on Chromium 65 and earlier -- Chromium 66 supports
native user styles and was first released more than two
years ago.

In Chromium-based browsers, the pseudo user styles code
is being unconditionally injected in every page/frame
just in case the browser is version 65 or earlier.

Removing pseudo user styles reduce uBO's main content
script in Chromium-based browsers by more than 20K.

Related thread:
- NanoAdblocker#348 (comment)