Remove obsolete/harmful overriding of documentation CSS

I haven't noticed any effect of the `.page-tools` part of the CSS code.
927406cc introduced this code in 2014 -
it must have become obsolete at some point after that.

Forcing white body background affects the navigation menu at the bottom
of most documentation pages. I find light-gray text on white background
significantly less readable than the default light-gray on black.
Perhaps the situation was different in 2016 when
ec8ca05f forced the white background.

This overriding of CSS has not been working for 5 years since
08383711, so I don't think anyone would
miss it.