Remove unused AdminPage and related components

Remove `AdminPage` and any components that are *only* used by `AdminPage`, and then only handle the `injectIntl` -> `useIntl` in the components that are still in use.

_Originally posted by @brian-smith-tcril in https://github.com/openedx/frontend-app-admin-portal/pull/1635#discussion_r2331403548_
            