Use BSD tools in scripts/clean

What does this MR do?

Unix platforms like macOS/OSX use BSD tools. The following commands had been written for GNU tools:

• readlink -f
• rm --recursive --force This MR replaces these two occurrences in the script scripts/clean by BSD tools supported commands.

How confident are you it won't break things if deployed?

Works on macOS default terminal using /bin/bash. Might by tested on other (Linux) platforms.

Links to related issues

The readlink issue had been addressed in MR !416 (merged)

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))