Remove unused markdown-it-emoji dependency

[Rob--W]

markdown-it-emoji was added 8 years ago because rules.md contained markdown emojis to signal the status of implementation, in 5c03c0b

4 years ago, these markdown emojis were erased from the rules.md file by b4f1107

Today, rules.md does still not contain markdown emojis, which you can verify by searching for : within docs/rules.md and confirming that there are no emojis (only URLs).

Therefore we can remove the package from our devDependencies and scripts/build-rules file.

┆Issue is synchronized with this Jira Task