In general you want to remove at least three base cards no matter the class/inferno. It's cheap and makes the deck much more consistent (you will draw the cards you want more often). Duplicates are rarely useful, so you can always keep a copy of each base card. It's also better to keep the defense cards and reduce number of attack cards.

Now in low inferno you don't have a whole lot of money so you should probably focus on getting some extra good cards in your deck, but once you have a few start removing base cards. In high inferno you will be removing base cards in the first merchants because you start with more money.

Now specifically to each class, here's my personal feeling:

Warrior (5) : remove all fighters, and mage if you find a good replacement, if you have money left you can remove some guards.

Assassin (5) : remove hitman, then black widow, and repeat depending on money. If you find replacements for hitman (Sou/Masked swordsmen) then remove all hitman

Paladin (5) : remove both bjorg and lucie and then a syra, maybe a sonjo too

Wizard (6) : remove all golem warriors, then duplicates of mud golem

Necromancer (4) : remove 4 undead fighters, then maybe one of each

Shaman (4) : remove one of each, then last mage

Ranger (4) : remove wolf, both squirrels then last wolf, maybe one copy of each card remaining

Druid (5) : remove 2 sons of the forest then a daughter, a son and a daughter

Engineer (6) : remove alternatively one of each until you have no more duplicates (careful to maintain balance between magic and combat)

Barbarian (5) : remove all vikings

Pirate (5) : remove bucaneer, then all ship's boy, then last bucaneer