Remove redundant styling props in the next major version of xstyled

[quantizor]

There are a number of redundant props: bg vs backgroundColor, maxW vs maxWidth, maxH vs maxHeight

On the surface this doesn't seem like a big deal, but if you're making a component library using xstyled which has some base styling and allows overrides by the consumer, this could lead to issues if they don't match using the exact same prop.

We should choose to favor either the verbose or short versions and deprecate+remove the others to reduce complexity.

[jguddas]

Can you share an example where overriding does not work?

Doing it like this for example works just fine.

[gregberge]

@probablyup I see what do you mean. It is a breaking change but I should consider it!