Tweak how mm-display-external handles windows

* lisp/gnus/mm-decode.el (mm-display-external): Don't delete other
buffers when displaying parts (bug#22861) because that should be
up to the user, probably.
1 job for master in 53 minutes and 35 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Test
failed #2782
test-all

00:53:35

 
Name Stage Failure
failed
test-all Test
Makefile:318: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
make[2]: *** [check-doit] Error 1
make[1]: *** [check-expensive] Error 2
Makefile:293: recipe for target 'check-expensive' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
make: *** [check-expensive] Error 2
Makefile:959: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1