fix(material-experimental/mdc-dialog): using incorrect mixin for structural styles

[crisbeto]

The MDC dialog was using the core-styles mixin instead of the static-styles which is supposed to be used together with the new theming API. I've also removed the container-elevation and container-shadow-color variables from the theme since they weren't doing anything.

[crisbeto]

cc @mmalerba. I'm fine with delaying this one until #25352 lands to reduce the amount of merge conflicts.

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