Don't use a hashtable in gnus-backlog

* lisp/gnus/gnus-bcklg.el (gnus-backlog-articles): Remove
  gnus-backlog-hashtb, which wasn't doing anything. Just keep a list
  of ident strings in gnus-backlog-articles.
  (gnus-backlog-setup): Delete unnecessary function.
  (gnus-backlog-enter-article, gnus-backlog-remove-oldest-article,
  gnus-backlog-remove-article, gnus-backlog-request-article): Alter
  calls accordingly.
1 job for scratch/gnus-hashtables in 23 minutes and 54 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
failed #738
test

00:23:54

 
Name Stage Failure
failed
test Test
Makefile:301: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
make[2]: *** [check-doit] Error 1
Makefile:271: recipe for target 'check' failed
make[1]: *** [check] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:955: recipe for target 'check' failed
make: *** [check] Error 2
ERROR: Job failed: exit code 1