Exiling the creature card with scavenge is part of the cost of activating the scavenge ability. Once the ability is activated and the cost is paid, it's too late to stop the ability from being activated by trying to remove the creature card from the graveyard.

If you activate Bannerhide Krushok's reinforce ability during your main phase, your opponents will have the chance to respond to that ability before you can activate the scavenge ability, perhaps by removing Bannerhide Krushok from your graveyard. However, if Bannerhide Krushok ends up in your graveyard during your main phase in such a way that there are no spells or abilities on the stack, you'll have priority to activate the scavenge ability before anyone can remove Bannerhide Krushok from your graveyard.