Delete unused mappers module.

[rtibbles]

Summary

• Any usage of this was already cleaned up, so this just deletes the module itself and any reference to it.

References

Fixes #8451

Reviewer guidance

If the build still works, this is fine to merge.

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.18.0b1.dev0_git.140.g792fc014.pex
Windows Installer (EXE)	kolibri-0.18.0b1.dev0+git.140.g792fc014-windows-setup-unsigned.exe
Debian Package	kolibri_0.18.0b1.dev0+git.140.g792fc014-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.18.0b1.dev0+git.140.g792fc014.dmg
Android Package (APK)	kolibri-0.18.0b1.dev0+git.140.g792fc014-0.1.4-debug.apk
Raspberry Pi Image	kolibri-pi-image-0.18.0b1.dev0+git.140.g792fc014.zip
TAR file	kolibri-0.18.0b1.dev0+git.140.g792fc014.tar.gz
WHL file	kolibri-0.18.0b1.dev0+git.140.g792fc014-py2.py3-none-any.whl

[marcellamaki]

quick smoke test confirm cleanup looks good