[mpora] Remove extractor

[ghost]

Please follow the guide below

• You will be asked some questions and requested to provide some information, please read them carefully and answer honestly
• Put an x into all the boxes [ ] relevant to your issue (like this: [x])
• Use the Preview tab to see what your issue will actually look like

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Make sure you are using the latest version: run youtube-dl --version and ensure your version is 2017.07.30.1. If it's not, read this FAQ entry and update. Issues with outdated version will be rejected.

• I've verified and I assure that I'm running youtube-dl 2017.07.30.1

Before submitting an issue make sure you have:

• At least skimmed through the README, most notably the FAQ and BUGS sections
• Searched the bugtracker for similar issues including closed ones

What is the purpose of your issue?

• Bug report (encountered problems with youtube-dl)
• Site support request (request for adding support for a new site)
• Feature request (request for a new functionality)
• Question
• Other

There is no more videos on https://mpora.de/videos/ anymore, lile in https://web.archive.org/web/20160116163404/http://mpora.de/videos/
Site changed completly and on some articles there are videos from youtube: https://mpora.de/articles/voyages-of-agape-70-000-kilometer-lange-reise-mit-dem-segelboot-um-die-welt.html