Remove incorrect (and inappropriate) comment in dprintf_dnode

Remove incorrect (and inappropriate) comment in dprintf_dnode

This comment seems to misunderstand the ## preprocessor token, which
does token concatenation.  It is not needed here, since we are
concatenating string literals, which is performed by putting the
literals next to each other.

Additionally, the comment uses offensive language.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #8698
Closes #8699