Remove unused check from dmu_tx_count_write()

Individual transactions may not be larger than DMU_MAX_ACCESS.
This is enforced by the assertions in dmu_tx_hold_write() and
dmu_tx_hold_write_by_dnode().  There's an additional check in
dmu_tx_count_write() however it has no effect and only sets a
local err variable.  We could enable this check, however since
it's already enforced by ASSERTs elsewhere I opted to remove it
instead.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3731 
Closes #11384

module/zfs/dmu_tx.c

@@ -230,9 +230,6 @@ dmu_tx_count_write(dmu_tx_hold_t *txh, uint64_t off, uint64_t len)
 
 	(void) zfs_refcount_add_many(&txh->txh_space_towrite, len, FTAG);
 
-	if (zfs_refcount_count(&txh->txh_space_towrite) > 2 * DMU_MAX_ACCESS)
-		err = SET_ERROR(EFBIG);
-
 	if (dn == NULL)
 		return;