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

include/sys/dmu_impl.h

@@ -173,7 +173,6 @@ extern "C" {
  * 	dn_notxholds
  *	dn_nodnholds
  * 	dn_dirtyctx
- * 	dn_dirtyctx_firstset
  * 	(dn_phys copy fields?)
  * 	(dn_phys contents?)
  *   held from:

include/sys/dnode.h

@@ -344,7 +344,6 @@ struct dnode {
 	kcondvar_t dn_notxholds;
 	kcondvar_t dn_nodnholds;
 	enum dnode_dirtycontext dn_dirtyctx;
-	const void *dn_dirtyctx_firstset;	/* dbg: contents meaningless */
 
 	/* protected by own devices */
 	zfs_refcount_t dn_tx_holds;