Remove unused KResponsiveWindowMixin.

[rtibbles]

Summary

• Removes accidentally retained KResponsiveWindowMixin

Reviewer guidance

This didn't seem to affect anything, and there were no used properties in the page.

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Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-.pex
Windows Installer (EXE)	kolibri-0.16.0a0.dev0+git.770.gee921f9e-unsigned.exe
Debian Package	kolibri_0.16.0a0.dev0+git.770.gee921f9e-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0a0.dev0+git.770.gee921f9e-0.4.0.dmg
Android Package (APK)	kolibri-0.16.0a0.dev0+git.770.gee921f9e-0.1.0-debug.apk
TAR file	kolibri-0.16.0a0.dev0+git.770.gee921f9e.tar.gz
WHL file	kolibri-0.16.0a0.dev0+git.770.gee921f9e-py2.py3-none-any.whl

[nucleogenesis]

❤️ thanks for this LGTM