dnode: remove dn_dirtyctx_firstset

Old debug param, not used for anything.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Robert Evans <evansr@google.com>
Reviewed-by: Adam Moss <c@yotes.com>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16297
Closes #17652
Closes #17658