Don't change the default of nnmail-file-coding-system

* lisp/gnus/nnmail.el (nnmail-file-coding-system): This governs how
  articles are decoded and saved, nothing to do with group names,
  shouldn't have touched this.
1 job for scratch/gnus-decoded in 50 minutes and 11 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #1521
test-all

00:50:11