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

docs/KNOWN_BUGS

@@ -82,15 +82,6 @@ may have been fixed since this was written!
   connect phase to most people. Brought up (and is being misunderstood) in:
   https://curl.haxx.se/bug/view.cgi?id=856
 
-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
-
 67. When creating multipart formposts. The file name part can be encoded with
   something beyond ascii but currently libcurl will only pass in the verbatim
   string the app provides. There are several browsers that already do this