fix(material/button): add comment to say raised-button is unused

[andrewseguin]

The M2 palette had a background key for raised-button that used to be applied for some button variants, but for a while now we've just used the "card" color. Adding a note next to the key saying it's unused

[crisbeto]

LGTM although the only way this would be surfaced to users is looking at the source code.

[andrewseguin]

Yeah, it's more for the sake of documenting than anything. It'll stick around to not be breaking, but I think we should avoid adding it to anywhere (which I doubt we'll ever want to do)

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}