I think you are confusing card removal with the dealer’s up card. The following table shows the player’s expected value according only to the dealer’s up card, before the dealer checks for blackjack, assuming the dealer stands on soft 17 and infinite decks.
Expected Value by Dealer Up Card
Up CardExpected Value 29.10% 312.38% 415.85% 519.65% 623.40% 714.40% 85.82% 9-4.06% 10-17.36% A-33.78%
So it is indeed true that if you could choose an up card for the dealer it should be a six. While sixes are bad for the dealer they are bad for the player as well. All things considered, if you could remove one card from the shoe you should choose a five.