Removal of sepia and night modes

[JayPanoz]

We removed day mode since it was confusing in the new theming system of version 2, given the flag was non-functional and day mode simply set default colors.

While implementing version 2 in Readium/Thorium, we noticed that we would probably not be using these modes in the various toolkits and apps, but use presets of the theming custom properties. In Thorium Web for instance we are just using the colors that are exposed through json, and extend theming from that because we need far more. So we are not using the flag any longer in the Preferences API.

This raises the question whether we should not remove these modes entirely, as they would only be present in version 2 out of courtesy as consumers that may not be even using them.

[JayPanoz]

Note this would also allow to remove the a11y media queries module, as it's barely used for customization and is not exposed.

[JayPanoz]

Proposed resolution: we will remove these modules, as discussed during yesterday's engineering call.

@danielweck since you could not attend and are one major consumer, do not hesitate if you can see negatives in this proposal, I will not resolve immediately.

To sum things up Web and Mobile are migrating to the extended theming capabilities to create their own, maybe with presets to ease usage a little bit, and will no longer be using sepia and night modes directly from Readium CSS. We consequently deemed it too much burden to maintain modules we will no longer actively use ourselves.