dynamic_debug: Remove unnecessary __used

The __used attribute prevents gcc from eliminating
unnecessary, otherwise optimized away, metadata for
debugging logging messages.

Remove the __used attribute.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Jason Baron <jbaron@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>