Remove mostly useless warning against using npm

[rtibbles]

Summary

• A long, long time ago, in a galaxy far, far away, we added a preinstall script to prevent people using npm to install dependencies.
• Due to a bug in NPM from v7 onwards that devs show no inclination to fix [BUG] Preinstall script runs after installing dependencies npm/cli#2660 this now only runs after all dependencies have been installed. It's possible that this could still be useful, but after we saw a false positive for the check, where someone running yarn was prevented from installing dependencies, it seems better to just remove it as it is mostly useless.

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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-0.16.0b10.dev0_git.70.g57334349.pex
Windows Installer (EXE)	kolibri-0.16.0b10.dev0+git.70.g57334349-unsigned.exe
Debian Package	kolibri_0.16.0b10.dev0+git.70.g57334349-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0b10.dev0+git.70.g57334349-0.3.0.dmg
Android Package (APK)	kolibri-0.16.0b10.dev0+git.70.g57334349-0.1.0-debug.apk
TAR file	kolibri-0.16.0b10.dev0+git.70.g57334349.tar.gz
WHL file	kolibri-0.16.0b10.dev0+git.70.g57334349-py2.py3-none-any.whl

[marcellamaki]

seems fine to me! merging into 0.16.x so it can be updated before new kolibri-tools version is released