remove reference to a missing "download all microdescs" feature.

   need to fetch the microdescriptors that have changed.

   When a client gets a new microdescriptor consensus, it looks to see if

   there are any microdescriptors it needs to learn.  If it needs to learn

   more than half of the microdescriptors, it requests 'all', else it

   requests only the missing ones.  Clients MAY try to determine whether

   the upload bandwidth for listing the microdescriptors they want is more

   or less than the download bandwidth for the microdescriptors they do

   not want.

   [XXX The 'all' URL is not implemented yet. -KL]

   there are any microdescriptors it needs to learn, and launches a request

   for them.

   Clients maintain a cache of microdescriptors along with metadata like

   when it was last referenced by a consensus, and which identity key