fix(cdk/overlay): remove deprecated variables

Removes some variables in the CDK overlay that have been deprecated for a while and don't appear to be exposed anywhere.

BREAKING CHANGE:
* `$z-index-overlay-container` has been removed. Use `$overlay-container-z-index` instead.
* `$z-index-overlay` has been removed. Use `$overlay-z-index` instead.
* `$dark-backdrop-background` has been removed. Use `$overlay-backdrop-color` instead.
* `$z-index-overlay-backdrop` has been removed. Use `$overlay-backdrop-z-index` instead.

Author: crisbeto