Remove KResponsiveWindowMixin

[MisRob]

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Summary

KResponsiveMixin has been superseded by the useKResponsiveWindow composable, which is the preferred way of accessing reactive window size information.

The goal of this issue is to remove KResponsiveMixin from the KDS codebase once and for all.

Guidance

• Studio and Data Portal don't seem to use KResponsiveWindowMixin at all, and in Kolibri, we've migrated all its occurrences to useKResponsiveWindow already (Replace KResponsiveWindow mixin by useKResponsiveWindow composable kolibri#11321), so it shouldn't cause any issues. However, when installing the release with this update, it'd be best to double-check just in case there are some places I haven't noticed.

Acceptance Criteria

• lib/KResponsiveWindowMixin.js no longer exists
• All references to it are cleaned up from the code or from the documentation, if there are any

[MisRob]

Closed by #719