src/usermod.c: Remove 'no changes' informative output

src/usermod.c: Remove 'no changes' informative output

No news is good news.

Debian needs to parse this message to ignore it, or alternatively check
if the call will be a no-op (which we already do) and skip the call.
If we remove this output, we're allowing Debian to remove that
complexity in their wrapper.

We don't expect this output to be very useful for interactive use
either.

Also, this message was changed from stderr to stdout recently, so we
don't need to worry about old scripts that might break due to this
change.  If there were scripts relying on that, they would have been
broken already in the previous change.

Closes: <https://github.com/shadow-maint/shadow/issues/1361>
Reported-by: Marc Haber <githubvisible@zugschlus.de>
Cc: <https://github.com/cachius>
Signed-off-by: Alejandro Colomar <alx@kernel.org>