maint: remove ‘coding: utf-8’ on ASCII

maint: remove ‘coding: utf-8’ on ASCII

These days, ‘coding: utf-8’ tags aren’t needed in general,
as Emacs typically defaults to UTF-8, and Emacs itself now
has many untagged source files in UTF-8.  However, for now let’s
just remove the tags when they are on ASCII files.
* lib/md5-stream.c, lib/sha1-stream.c, lib/sha256-stream.c:
* lib/sm3-stream.c: Remove unnecessary coding: lines.