not really a need to remove mana potions, the new potions don't provide enough mana regen to make them worth buying regardless of them refilling at base. some champions are really mana dependant and require extra mana to be relevant in lane.
"In a world without mana potions (and generally less early mana regeneration), we've identified a few champions for some emergency rations, giving early mana boosts until they can get their footing and purchase items to make up the deficit. This also means we can really understand which champions relied on mana potions as a crutch to limp through the early to mid game, and we can give additional love if necessary." Source: The patch notes.
A more full discussion if you're interested: "Mana Potions have been removed from the game. We’ve generally found that the decision to opt into Mana Potions was one that was either forced upon some champions (aka “I need this mana to function”) or one that was generally only available to a handful of circumstances. The best use cases of Mana Potions were typically to circumvent Mana restrictions for boom/bust play patterns (ie: save up to X amount of mana before dumping it all in a large burst of damage, repeat) vs. attrition-based approaches that are more in favor of straight mana regeneration. A champion being able to access their big “all-in” combo more frequently also makes it challenging for us to balance appropriately, and we’ve seen some more frustrating champions have their power reduced as a result. With the removal of mana potions, we can also retune some champion mana values back to a middle ground. We’ve looked at the data for champions that frequently bought 2 or more mana potions in the game and we’ve either increased their maximum mana or the mana regeneration values based on their class."
