[ie/ORFTVthek] Remove extractor

[seproDev]

IMPORTANT: PRs without the template will be CLOSED

Description of your pull request and other information

The ORF TVthek has been entirely decommissioned and replaced by ORF ON. https://www.derstandard.de/story/3000000220844/orf-dreht-tvthek-endgueltig-ab

Template

Before submitting a pull request make sure you have:

• At least skimmed through contributing guidelines including yt-dlp coding conventions
• Searched the bugtracker for similar pull requests
• Checked the code with flake8 and ran relevant tests

In order to be accepted and merged into yt-dlp each piece of code must be in public domain or released under Unlicense. Check all of the following options that apply:

• I am the original author of this code and I am willing to release it under Unlicense
• I am not the original author of this code but it is in public domain or released under Unlicense (provide reliable evidence)

What is the purpose of your pull request?

• Fix or improvement to an extractor (Make sure to add/update tests)
• New extractor (Piracy websites will not be accepted)
• Core bug fix/improvement
• New feature (It is strongly recommended to open an issue first)

[Hrxn]

orf:on is listed in supportedsites.md, so I assume this new site is already supported?

[seproDev]

@Hrxn Yes. You might need to update to nightly for the latest improvements to the orf:on extractor.
If you experience any issues, please do not comment on already merged PRs, but open a new issue.