What should I take out? Khans deck

First off, is this deck for block constructed, standard?  What format is this meant to be?  Second, what type of setting are you playing in?  Are you looking to rock the kitchen table circuit or are you thinking this is ready for FNM?

From a quick, cursory glance, I can see MAJOR problems with it that would stop it from being good at FNM, but I'll hold off on all of that until you give a sense of where this thing is going.

One quick pointer though, given the amount of fixing in Khans, you really should be running some of the dual or tri-lands (even if you don't have the temples, painlands, or fetches).

Is it for one-on-one or multiplayer?

Okay, so a few changes to be made and cards to cut:

Rakshasa's Secret is pretty bad.  Against an aggressive deck, they've already dropped their hand before you can really cast this.  Against a control deck, their hand will be full and since they get to choose which two cards to discard, well, they can keep their big threats.  If you're looking for affordable hand-disruption then Despise is the place to be looking.  Obviously Thoughtseize is the gold standard, but if you don't want to drop the cash for them (or don't have them already) then that's $80 worth of four cards...

I get that you're trying to fill the yard, but Bitter Revelation strikes me as a strictly worse Sign in Blood or Read the Bones.  I would say to run one of those and use other cards to fill the yard.

Death Frenzy is just worse than Drown in Sorrow and I would say is even worse than other removal as it will end up taking out all your zombie tokens (not exactly an ideal solution).  I'd honestly cut these for a few Sultai Charms or more general removal.  Sideboard Drown for aggro match-ups, but otherwise, this has no business in the main.

3x Liliana Vess strikes me as too high.  I'd say only run 2x of her at the most.

4x Rakshasa Vizier also strikes me as being too high.  Perhaps cut this down to 3x.

Sultai Soothsayer comes across as a worse version of Satyr Wayfinder, Taigam's Scheming, or Commune with the Gods when it comes to filling the yard.

Shambling Attendants are just bad.  I get that you're looking to use Delve, but there are better cards to be casting with Delve than this.  Treasure Cruise is an easy example as, in the mid-to-late game where you have 7+ cards in the yard, it essentially becomes Ancestral Recall (which is pretty good I hear).

I don't know that I'd run 4x Necropolis Fiend either.  Perhaps as a 2x.  He's a bomb, but in reality is mostly just a 4/5 flyer that will come down for cheap here and there.  His ability isn't something you'll be using all that much.

So, the long story short, delve is a neat mechanic and certainly one with promise, but keep in mind that you only have so many cards in the yard to use with delve.  Once they're used, many of the delve cards are alarmingly overcosted, meaning you need to pick and choose which delve cards to use carefully.

So that being said, I would make the following changes if I were running the deck:

-4x Rakshasa's Secret
-4x Bitter Revelation
-1x Nyx Weaver
-4x Death Frenzy
-2x Liliana Vess
-2x Murderous Cut
-1x Rakshasa Vizier
-4x Sultai Soothsayer
-4x Shambling Attendants
-2x Necropolis Fiend

Total: -28 cards
Needs: 15 cards (as you were 13 over to begin with)

+3x Treasure Cruise
+2x Sultai Charm
+2x Hero's Downfall
+3x Dakra Mystic (it can let you trigger Sidisi one card at a time while also controlling the opponent's draws if they have a Courser)
+2x Sign in Blood
+2x Commune with the Gods or Taigam's Scheming (I'd go commune, but that's me)
+1x Whip of Erebos (lifelink and you can USE the creatures in your yard for more than just delve)

If my math is right, that will put you up at 60 cards and should give you a lot better feel and power for the build.  It might be casual play, but it's always nice to have a deck run smoothly.  Good luck!