KNOWN_BUGS: remove 68, 70 and 72.

Due to their age (we don't fully know if they actually remain) and lack
of detail - very few people will bother to find out what they're about
or work on them. If people truly still suffer from any of these, I
assume they will be reported again and then we'll deal with them.

72. "Pausing pipeline problems."
  https://curl.haxx.se/mail/lib-2009-07/0214.html

70. Problem re-using easy handle after call to curl_multi_remove_handle
  https://curl.haxx.se/mail/lib-2009-07/0249.html

68. "More questions about ares behavior".
  https://curl.haxx.se/mail/lib-2009-08/0012.html