md-content: Remove md-padding class in favor of layout-padding

[epelc]

I'd like to suggest we remove the md-padding class for md-content as it is essentially the same as the layout-padding attribute. It seems like it might confuse people and having them try to use md-padding on other elements when we already have layout-padding and layout-margin for that.

[JoeyHoutenbos]

Totally agree this is pretty confusing, especially because md-padding can only be used on md-content.

[ThomasBurleson]

@rschmukler - rename md-padding to md-content-padding or use the layout-xxx?

[marcysutton]

.md-padding was already renamed from .md-content-padding a few months ago, so I'd advise against doing that. Sounds like the attribute method is the way to go.