Remove outdated info from download.html

Hi there,

This removed outdated information from download.html.

• http://hackage.haskell.org/trac/ghc/wiki/Builder is an outdated link to https://gitlab.haskell.org/ghc/ghc/-/wikis/builder which describes an outdated system and links to https://gitlab.haskell.org/ghc/ghc/-/wikis/continuous-integration which also describes an outdated system :D <3 We could make it point to somewhere on GitLab but I could not find the right place.
• The ghc-builds mailing list has not been in use since August 2017
• http://haskell.inf.elte.hu/builders/ doesn't exist anymore
• AFAIK the PPA in Launchpad is not supported anymore

Just removing all this is admittedly a coarse stroke, in the sense that some information could be replaced by more relevant one. But it is a starting point and I'd be happy to revise this PR in a second pass.