configure: Remove stray debug output

Rather than printing a message saying we're silently falling
back to gthread coroutines when running on MacOS, actually
do it silently.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

configure

@@ -2969,8 +2969,6 @@ EOF
     else
 	coroutine_backend=gthread
     fi
-  else
-    echo "Silently falling back into gthread backend under darwin"
   fi
 elif test "$coroutine" = "gthread" ; then
   coroutine_backend=gthread