Remove unused messages

It looks like they are a copy&paste rest.
At least some of the removed messages a duplicate of
https://translatewiki.net/wiki/Special:Translate/ext-wikiforum?filter=&action=translate

If I am wrong and these messages are needed, please change the message key
to avoid duplicate message keys with the WikiForum extension

Change-Id: I2956bed83b62e380fcb5a90c011a4057bf10e6db
Merge "Remove unused messages"