Fix Gnus registry header extraction

Fix Gnus registry header extraction

* lisp/gnus/gnus-registry.el (gnus-registry-fetch-recipients-fast,
  gnus-registry-fetch-sender-fast): First, delete
  `gnus-registry-fetch-header-fast'. It was being called with reversed
  arguments, and thus always returned nil, but even if the argument
  order was correct it would have raised an error, as it was trying to
  `assq' a string in a vector. Instead, just have these two functions
  do their own work, as they're doing fairly different things.