refactor: Remove MDC theme's deep-get, used sass:map's get API instead.
refactor: Remove MDC theme's deep-get, used sass:map's get API instead.

BREAKING CHANGE: Removed `deep-get()` API from mdc-theme, use `sass:map`'s get() API instead.

PiperOrigin-RevId: 345257138

material-components/material-components-web,material-components/material-components-web,material-components/material-components-web