Duplicated CSS imports

[alanxone]

https://github.com/angular/material2/blob/89cad3fd1511def6ea9aa17c867f9872bf774f2c/src/lib/menu/menu.scss#L3

https://github.com/angular/material2/blob/89cad3fd1511def6ea9aa17c867f9872bf774f2c/src/lib/menu/menu.scss#L5

Why importing the same file twice?

[FrederikSchlemmer]

Seems like one import isn't needed.

@AndrewTW Would you like to make a PR? Otherwise I would do this.

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