With new sets bring new cards. And MH sets bring very good cards for cube.

White:

Scion of Vitu-Ghazi isn't a bad card, and is often a solid pick 4-8. However, with the push towards flicker, Scion sends the wrong signals as the ability only functions when cast from hand. As such, it's coming out. Grand Master of Flowers will be the replacement, acting as a planeswalker version of Icy Manipulator.

Sunscour, on the other hand, is a card that barely sees play. It's just too expensive. The upside of being able to freecast a board wipe is usually not worth the 2 cards from your hand, if you even have 2 white cards to pitch. There just aren't enough game ending early threats for the benefits to outweigh the costs. Perhaps someday, but not today. Instead, it has been replaced with Out of Time as the undercosted white boardwipe with drawback.

Multicolor:

For more support for the Jund sacrifice archetype, Ravenous Squirrel has been added. Sadly, this means we need a cut from the already existing Golgari cards, in this case it was Garruk, Apex Predator. There is always a chance he will return though.

I haven't been liking the play patterns that Ashiok, Nightmare Muse has been promoting, so Ashiok gets to take some time on the bench. Master of Death will be tagging in for now, as support for generic good decks, as well as graveyard strategies in general, with added benefits for cards like Survival of the Fittest.

Colorless:

The ever present Rupture Spire just isn't cutting it anymore. Setting yourself back two turns just isn't worth the perfect colors anymore. Thankfully we have a plethora of other options to take advantage of. In this case, Fabled Passage is the card taking the slot.