do not rely on 0 plugins for detecting headlesschrome

[unixfox]

Hello,

I'm running BotD in production and with one of my user I diagnosed that navigator.plugins.length returns 0 on his browser.

He is running Vivaldi 7.0.3495.18 (Stable channel) on Windows and without any extension installed the browser gives no plugins in the list:

I'm proposing to remove the classification that if the plugins list is empty, then it's a headless chrome.

First because now for --headless=new, the value for navigator.plugins.length is not empty anymore. It's 5. Everyone is already using that new headless method or spoofing the value for navigator.plugins

And second, the API navigator.plugins is no longer recommended: https://developer.mozilla.org/en-US/docs/Web/API/Navigator/plugins. So we may run into some false positive if some browser decides to not disclose the plugins list as part of protecting the privacy of the user.