Removed obsolete threshold values

[patorjk]

What: Documentation for threshold was updated to remove parameters that don't exist anymore.

Why: These parameters don't exist anymore and the docs for the threshold are a little confusing.

How: Parameters removed from documentation.

Checklist:

• [ X ] Documentation
• [ ] Tests N/A
• [ X ] Ready to be merged

I was reading through the documentation for this on npm and was confused by what the STRING_CASE and STRING_CASE_ACRONYM values for threshold were. I checked the code and they weren't in there. I searched the repo and they only show up in the README and in a very old PR. It looks like they used to be used a long time ago but at some point got removed.

[github-actions[bot]]

:tada: This PR is included in version 6.3.2 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: