refactor(material/sidenav): undeprecate constructor signature

[crisbeto]

Removes the deprecation labels from a constructor parameter since it's tricky to
sync into g3 and is easy to maintain on our end.

[wagnermaciel]

@crisbeto Please resolve this merge conflict

[crisbeto]

Rebased.

[annieyw]

Apps using <mat-sidenav> without a container would have trouble making this change

[crisbeto]

I was under the impression that we require sidenavs to be inside a container. If that's not the case we can un-deprecate the signature. Can you weigh in @mmalerba?

[jelbourn]

@crisbeto this came up because someone is actually doing it- we found an instance where mat-sidenav is the root element in a component. We'll have to fix them before we can move this ahead

[crisbeto]

I was asking, because I had the impression that we needed a sidenav inside a container, but if that's not the case and we want to support it, keeping the parameter optional won't be a big deal.

[jelbourn]

I don't feel strongly either way; it certainly was never intended to work, but it's probably not worth the effort to fix it

[crisbeto]

Okay, in that case I'll revert the changes and turn this PR into a merge-safe one that un-deprecates the old signature.

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